If it was me, I would set them both up outside a case. This way I would be able to determine wich one is the best for the job.
Swap cards around, do some benchmarks and check how it will perform with cache disabled.

For DOS I'd use the Intel AN430TX and the 200mhz p1 cpu.
Maybe couple it with another 2D vga, like an S3 (get a name one if possible since it should have better parts than chinese cards) since it will more compatible with DOS stuff.
The millenium has too many red blocks for my taste: http://gona.mactar.hu/DOS_TESTS/

My DOS ss7 build is a 233mhz mmx + via mvp3 mobo (HOT-591P) + S3 Trio3D/2x vga + SB16 CT2230 + MPU + GUS classic. It covers everything I want, from really old stuff to DOS software 3D games. Using ICACHE to disable caches on the fly brings it down to 386DX speed levels. Phil has some great videos on the ss7 situation too

I do also have a Trident 9440 PCI card, which seems to be mostly green on that site. I do also have some S3 Virge somewhere too, but I havent dug it out yet.

Ah, the Virge is a good one - provided it has a nice signal output. There is a chance the image is too bright but there is a TSR now that fixes that: Re: S3 AGP Cards (and possibly others) Too Bright

Haven't used a PCI Trident card so I can't really comment on it - you can use Phil's benchmark suite to see which one is better 😀
http://www.philscomputerlab.com/dos-benchmark-pack.html

TX can only cache 64mb ram, HX can do more but only if it has the right cache stick (and a slot for it). 166mmx can do 200mhz without Any problems and is the better chip. At 166mhz it is almost as fast as the normal 200 chip.

Test register manipulation with SETMUL can be handy for tuning your effective CPU speed on older DOS titles. I'd check to see if your P166MMX exposes TR12 functionality (information is split). If not, it's another reason to consider using the P200.

See here for details:
Re: What's up with all those Setmul switches?