The points you will want to take into consideration are thickness, durability, traction and the ability to use both sides.
Thickness – You’ll want enough cushioning to last through the class. Especially in hot environments, many types of foam rubber tend to soften. This means that if you have a mat that isn’t thick enough, the softening of the material may not offer the same cushion that it might in a cooler place.
Durability – This point really goes for just about anything you buy, which will undergo continuous use. When it comes to Bikram or hot yoga, the constant use from hot to cool temps if you tend to switch up your style of practice. The sweat and constant need for cleaning are also factors that may affect the longevity of the mat as well.
Traction – Whereas you won’t want to have something that is gritty and uncomfortable, you will want to choose an option that provides an adequate grip, especially for Bikram where the heat is cranked up, causing you to sweat much more. though you may have to wipe the sweat off once in a while during a session, the better traction could mean ewer interruptions.
Reversible – Going back to the point above, you will tend to sweat a lot more with Bikram. For that reason, it is a good idea to find a product which is reversible, allowing it to simply be flipped over when too much sweat settles on it.