I really love my Olfa mat. I have a few of them from 24x36 down. I also have the two sizes of the rotating mats. They are great. I also use both the regular Olfa rotary cutter and the ergonomic one. I like the ergonomic 45" one. It is comfortable for long cutting sessions and I can maintain good control with that size. I always buy my cutters and mats at Joann's when I can use a coupon.
One day I would like to splurge and try Martelli's products. I don't think you can go wrong with Olfa - just be sure to use a coupon!