Is there something you want to play on the K6-2 in particular? Banshee is probably your best bet.

nothing particular, it is overshadowed by my P3 and offers no real advantage, but its IBM so i keep it around 🤣

The Banshee is the right choşce unless you are trying semi crazy things like running say GTA3 on a K6-2.. The 9250 is best suited for non AGP P3 systems like i810 and PLE133/T etc in my opinion..

Banshee is not right choice and sometimes will bottleneck even K6-2. But 3DNow! support on ATI cards is probably horrble anyway.

I like to pair Voodoo 3 cards with K6 systems, but even on my K6-III+ 550 that Voodoo 3 is bottlenecked hard most of the time. What's the clock on your K6-2? Ideally, a single Voodoo 2 card is probably all you'll ever need on a mid-clocked (~400-450MHz) K6-2, but of the two options you provide I'd just use the Banshee. The Banshee should be alright provided you play at 640x480 and maybe 800x600, but you'll probably won't play anything newer than 1997 games on the system anyway. It also has a really nice 2D chip, much like V3 and V4/V5 cards, with pretty good DOS compatibility and very sharp IQ.

3Dfx cards were often paired with K6 CPUs at the time, so 3Dfx put some really great optimization in their drivers, utilizing 3DNow! extensions and such, and because of this, unless the game you are trying to play can make use of HW T&L or is just too demanding for these cards, the 3Dfx cards will always be faster than Nvidia or ATi or Matrox cards on these systems thanks to lower CPU overhead and the ability to run games with Glide support.
At least that's what I've found out to be true in my experience throughout all these years.