I say get a 6" bench grinder. Most of the wheels out there are 6" and if you need parts, 6" is most available. Most of them look the same on the outside, and sometimes are the same on the inside. If its got ball bearings its worth the extra cash. Otherwise there are some others for about $45-$50 at Lowes that are pretty nice.
Angle grinders are always handy. Mines a 4.5" and I love it. Once again, ball bearings are a plus if its got them.