Voodoo 2 are more valued overall.

whichever one you had as a kid

I never actually owned a 3Dfx product before and never really cared for them. I have a chance to buy one for a decent price so I'll get one if I can.
Just the usual "I own a 3Dfx product!" reason. Strictly collection value, nothing more.

well idk what "collection value" means for you then ^^

if we go by purely monetary value a 12MB Voodoo2 is obviously worth more, unless one of the 8MB variants comes with box or is a special variant or something.

I think you mean is the "going rate", not the "collection value". The latter generally is whatever an object gets appraised at by a certified expert on a subject, usually for insurance purposes. In popular speech that usually gets interpreted by unscrupulous sellers as the amount they think they can extort from an unsuspecting buyer, whereas the "going rate" is what something usually sells for for real.

Looks at Ebay auctions that have ended and actually sold.

Alright, then:

Diamond Monster 3D II 8 MB PCI
or
A-Trend 3Dfx Voodoo2 12 MB PCI

?

well anything short of an original boxed (preferably new and/or sealed) mint card isn't even worth that much as to get looked at by any "expert" probably ^^

Eh, just couldn't do it.
$40 + shipping for an untested card is too much excitement for me.

I'm quite a Diamond fanboy, because the graphics cards generally have better components and nicer finish.