With the latest drivers the R8500 narrowly outperforms the Ti500 in most cases, and that was formerly the Ti500's only real definitive advantage.
R8500 has better 2D visual quality, better DVD playback, it has TV-Out and multi-monitor output natively, better TV-Out capabilities on the driver level, slightly better 3D feature set.
R8500 has faster but lower quality anisotropic filtering then the GF3, while the opposite is true for FSAA, in which ATi has the better visual quality but slower FSAA implementation.
ATi's drivers for the R8500 are pretty decent at this point in time, also... so I see no real reasons to choose a Ti500 over the R8500 anymore.