I have two Olfas, both 12x18 -- one is blue, the other is purple. I have an 18x24 Fiskars that is yellow on one side and lavendar on the other side. And I have another Olfa that is blue and 24x36 that stays on my cutting table. I also have a blue 6x9, two of the cutting/pressing combinations (one large, one small), and one 8x8 rotating cutting mat. What can I say -- I'm a gadget junkie.
Definitely get a self-healing mat. As quiltingcandy said, it is easier on the blades, and your mat will last longer.
You can look at art supply sites, which also carry self-healing mats. Sometimes they are cheaper than buying from a quilting site.