You did not mention how much money you are willing to pay.
If money is no object, you can get both. The Voodoo5 5500 comes in a PCI version as well.
If you play flight sims, etc. then the V5 5500 (with its FSAA) would be the better choice. If you do not care for FSAA, then the V5 5500 would be the better choice because it has better stable drivers.
Then again, you could get lucky and find out that every single game you play runs well without crashes on the Radeon while the V5 is the one with the stability problems. You won't really know for sure until you purchase both cards and compare.
For 2D work, the Radeon is the better choice (if you choose high resolutions on a pretty good size monitor 19+ inches). G400 and the Radeon are the best 2D cards you can get today.
If you do any sort of professional 3D work (apparently you don't since you didn't mention it), neither the 3dfx cards or the ones from ATi should be used at all. Their drivers stink in comparison to NVIDIA cards like the GeForce2 MX, etc. You do not want to have your work ruined because of idiotic drivers from 3dfx or ATi. Too risky.