In reply to humptydumpty:
Size & fold-type are the main issues, and these can both come down to preference.
Bigger can equal more confidence, but equally can be a pain to lay down if you're bouldering in cramped area. Smaller mats can look very small when you're freaked out on a high problem.
Fold types = Hinges fold flatter, so for taking on a plane might make more sense. Some people find the hinge area a bit of a risk area for landing though.
I think I'm right in saying any small/medium mat when folded will just about count as a suitcase for check-in. Large ones will probably count as sports equipment and attract a fee, or, if flying Ryanair, an organ donation.