There are a number of factors involved in selecting a yoga mat. The first is price. You can spend anything from about 20 dollars (actually I have seen economy mats where you can get to under 10 dollars a mat if you are buying a minimum number of mats) to over 100 dollars. How much you decide to spend will depend on how important various of the other factors are for you.
Dimensions aer relatively standard, though there is some variation. Some of the economy mats I have seen may be 68 inches long, though 71 inches is fairly standard, and I have seen long variations, like the Manduka long which is 85 inches. I believe Jade makes both a 68 inch and a 74 inch. A longer mat is good if you are tall, or if you really dislike having your toes or fingers hang over the edge of the mat when stretched out fully with your hands extended. Most mats are 24 inches wide, though I have seen a few 26 inches wide.
Mat thickness is likely to be more of an issue for more people. One problem in comparing is that some mats measure in millimeters and some in inches. Just remember that a quarter inch is a bit more than 6 millimeters, and this is probably the most standard thickness. Thinner (and typically cheaper) mats can be as little as 3 millimeters. Their only benefits are that they are often lighter and usually cheaper. Thicker mats can be as thick as a half inch, or 12.7 millimeters. The thicker mats are good for those who need a lot of cushioning, but they are heavier, harder to roll, and not as good for stability in standing poses. So most people will go for the 6 mm, though some for various reasons will go thinner or thicker.
Mat weight can also vary. Most mats will be about 5 pounds, though a few are heavier. For example the Manduka black is about 7 pounds.
The grip/slipperiness of a mat is also important. The thing is, every mat I have ever bought slipped more in the beginning, so I take reviews on this point with a grain of salt, unless they are reviewing at a fair period after buying the mat. If you have a natural rubber mat it has natural oils in it. Your skin also has natural oils. If you sweat the oils and the water of the sweat tend to make for slipperiness. There are a few things you can do on this front besides searching for the holy grail of a totally unslippery mat. First: wash your hands before coming to the mat. Also try not to have lotion on either your hands or feet. Second: especially if you are doing 'hot' yoga put a towel on top of your mat. Some companies sell matching towels sized to the mats. I have also seen some new products hat have dual sides, such as Aurora has a mat with a fused cloth side. I worry that this might be harder to clean without the sides separating, but I have no personal experience with the product. Third: some companies sell rosin, which can be wiped on the hands or on the mat where the hands and feet usually go, similar to what gymnists use. One company actually suggests rubbing the new mat lightly with salt. Salt, of course, tends to break down rubber, so I assume they want slightly to degrade the surfact to enhance grip. I would probably not do this, unless it was a really big problem, as I want my mat to last a long time, unless other things did not work.
Washability is another issue. Most mats can be washed in a washing machine, but should be air dried. There are also a number of spray products that are relatively inexpensive that can be used to clean and freshen the mat between washes. If you are going to use a mat other than your own it might be a good idea if you can afford to do so to buy a spray, rather than using whatever generic cleaner is provided at the studio/gym. I have personally gotten plantar warts twice from gym mats (and had them surgically removed, as I absolutely do not want to pass that gift on) and am very careful when using them now. I also know of a student who got an infection from one. Gyms are breeding grounds for bacteria and practicing good hygiene is really important.
Style is really a personal decision, but I just want to mention that some of the markings on the mat are about more than style. Some mats have a mark at the top middle of the mat. This is used as a tool for focusing and alignment. In the same way a mat with a vertical stripe helps with alignment.
The last issue is, in my mind, the most important: environmental friendliness. If a person does yoga strictly as an exercise, and has no other feeling about ever expanding trash dumps and the byproducts of toxic manufacturing, or if style trumps anything else, this paragraph will be moot. But my feeling is that for most of the students I have taught yoga is about more than body beautiful, but about self discovery, and self understanding, and about opening ones heart to oneself, one's community, and one's world. I have been working up to writing a piece on the ethical underpinnings of the discipline, but even without going into that deeply most people who do yoga get it. This is the three part mantra which I usually close class: to understand, to care, to act.For me this is basic: that the earth is our lifeboat and we must care for it. In any case, if you do want a natural rubber mat, with no foaming agents or toxins, and no footprint in landfills such products exist. And if you really need a mat, but just cannot afford the extra it costs that is understandable. Just remember if you have cable, or an x box, or a game boy, or whatever other things.... all of those things represent choices. I just think it is important to be clear whenever one buys anything that it is a choice.... and we should acknowledge those choices. And by the way, any mat that has PVC in it has heavy metals. You cannot make PVC without heavy metals, and even if you capture the heavy metals during manufacturing they have to be stored, as they are too toxic to put into the environment.
I say this because the Manduka pro mats claim to be very environmenally friendly but contain PVC. Their contention is that because their mats are well made they should last a lifetime and that will cut how much toxicity goes into landfills. They also claim no toxic waste ESCAPE in their manufacturing.... but that means there is toxic waste, but it is trapped and stored. But what happens when all the stored heavy metals spill? Or there is a storm? (remember the nuclear plant in Japan?) So all these issues are very complex, and you have to read very carefully before buying because anyone who wants to sell you something is going to tell you what you want to hear and put the stuff that might make you pause in very small print. Here is a link to an interesting blog on the subject. http://www.myfiveacres.com/go-green/is-your-yoga-mat-killing-you/ That blogger really liked the Lulumon mat. I have other ethical issues with the Lulumon company, but kudos to them if he is right about their mats.
Regarding brands: Gaiam mats are very popular and readily available. The 3 mm can be as inexpensive as 20 or 30 dollars, though the 5 mm will be more. The Manduka are often considered of the top line. They can go over 100$, but a regular 5 mm may be available for around 70 dollars. But see the last paragraph. I use an Aurorae. I find them comfortable, and reasonably environmentally friendly, and mine is slightly long, which is a feature I really like. The Jade mat is natural rubber, and goes usually for about 60 dollars, though they are also available in many places so it is likely one could get it cheaper. Prosource mats are very cheap, their advantage for those who need a lot of cushioning is that they are quite thick (half inch) but they are more environmentally unfriendly.
The bottom line is: Before you buy carefully consider what features work best for you. When you get your mat read the care instructions. Once you buy a mat take care of it, so it lasts as long as possible.
See you on the mat....