With miter saws, your options range from simple "cutoff saw" (just like it sounds) to sliding compound models (SCMS). The cutoff saw is the cheapest, only adjustable in one dimension, the traditional horizontal angle. The SCMS is the most versatile, giving you the ability to cut compound meters in variable width material. Brands range from cheap stuff like Harbor Freight or Black & Decker, to Makita, Hitachi, Bosch, Dewalt, etc.
It all really depends on what other stuff you might use it for. In my case, I do general construction and woodworking, so I always buy the most versatile and highest quality power tools. If that is you, I would recommend any of the big brand SCMS, but be prepared for big bucks!
If, on the other hand, you just want a cheap and easy way to easily cut pvc with a power saw, you can pick up a Harbor Freight cutoff saw for something like $50 if you shop sales or their coupons.
And if you are only cutting a few pieces of pvc and are looking for the cheapest solution, the hacksaw is the way to go!