I haven't used the others, so I can't compare. But I like the keys on the prime - they're hard rubber and make a satisfying click, and don't wiggle too much. Apparently the Prime-G1 had a contrast issue that affects visibility of the letters on the keys, but they fixed it in the Prime-G2.

Well it's worse than the hp48gx, and a bit better than the 49/50 models (well anything post 48gx).