In reply to rgirish28:
Having written the review, I stand by what I say about the Origin - it'd be a great first pad.
With regards to the others I've laid their respective dimensions out side by side, because that provides a bit of context behind what you're actually getting in real terms. Given that you don't buy a bouldering mat very often I would probably recommend buying the best, and maybe even the biggest, pad you can afford as you're going to get years of use out of it. Furthermore, even when you come round to buying a replacement you're likely to keep hold of it as a spare.
Origin: 100 x 134 cm / 11cm thick
Session: 91 x 121 cm / 10.2cm thick
Psychi: 90 x 120cm / 12.5cm thick
That said, if you're on a budget you're on a budget. It's been years since I used a Session, but I remember them being quite good. Haven't used the Psychi Pad, but at 12.5cm thick (which I assume is correct) that's quite a pad for the money!
Good luck...