The Retail R8500 is faster at stock clockspeeds, and can usually o/c to 300/300 to 315/315. The GF3 Ti200 can usually hit at least regular GF3 speeds, and sometimes a bit above GF3 Ti500 speeds.
The R8500 though has HydraVision, TV-Out and DVI Out by default, better FSAA, better 2D visual quality usually, a slightly better 3D feature set, and better anisotropic filtering performance, along with a more powerful T&L engine.
The only real advantage I see for nVidia is better driver support, but ATi has improved significantly in that regard while nVidia seems to be slacking off somewhat.
I might consider a Ti500 as a viable alternative over the R8500 Retail, but definitely not the Ti200.
Just make sure you use the more recent beta drivers for the R8500, as the earlier drivers were much slower and had definite compatibility issues.