My Voodoo 4 performs better with deeper and higher resolution screens but my Voodoo 3 with SGRAM is faster in memory and for some 2D operations. The extra memory is nice but the extra cost is not unless finding a bargain. The Voodoo 4 doesn't need an extra power supply like the Voodoo 5 so it is more practical. The Elbox 2D drivers have problems with big endian modes on the Voodoo 4/5 and the latest voodoo.card and pci.library don't work at all. A Voodoo 3 3000 with SGRAM is competitive with the Voodoo 4/5 in a Mediator and is one of the fastest and best 2D/3D gfx solutions for any classic Amiga.
Would have to disagree, even Doom/quake runs a lot better with the Voodoo 5 card over my Voodoo 3. Plus workbench runs smoother and I can get 1920x1080 with high true colour (or whatever it is) so if one can get a really good deal (which come up quite often on Amibay) I'd get the Voodoo 5.
The display also looks sharper, doesn't compare to the ATI card for 2D though.
I can also over clock the Vooodoo 5 higher than I could with the Voodoo 3 and the gains can be seen in frame rate. The latest drivers work fine for me. As for it needing additional power it's no different than plugging in a HDD so not really a good reason to not get a Voodoo 5 if you can.
So from a real world point of view Voodoo 5 ftw.
Currently I'm using 256MB of RAM from the Radeon 9200 as system memory and a 16MB from the Voodoo 5. Which proved to be the best combo for gaming 3d/2d performance.