I would expect the 440BX to be a lot better than a VIA MVP3 for I/O. I had a good experience with a 440GX 450MHz system doing video capture a long time ago, and that's really the same thing as a 440BX.
My system before that was a Tyan S1590 board like yours, but I never tried using it for an I/O heavy situation.
Main memory throughput is definitely faster with the 440BX. I believe it also has better PCI throughput, which might matter if you make heavy use of the PCI bus.
Intel P2/P3 has a faster FPU than K6 series.
K6-2+/3 do have faster L2 than Katmai though, so that raises a little doubt.
You could try both and do a comparison, but I'd be surprised if the winner wasn't the 440BX setup.
I wonder if a CuMine willrun in it? If so, I have a P3-667 that would run at 500 in it...
First thing to worry about with Coppermine is whether the motherboard's VRM supports those voltages. The chip that regulates voltage is probably about 20 pins and located near the CPU slot, near the MOSFETs, inductors and capacitors in that area. If you can read the part number on that chip (or take a photo) we might be able to look up what voltage range it supports.
If the voltage regulator is compatible, then you could plug in your Coppermine to see if it boots, which will tell you if the BIOS supports them. Even at 500/100 it should be faster than a Katmai 450MHz so you could just roll with that.
It's unlikely for the Katmai to be faster than the Coppermine, but it could happen if the application really wants the larger cache.
There's definitely more CPU upgrade options on the Slot-1 440BX board than there is on the S1590.