Okay, Liza and Ninuh, you have got me out of my element here, but I will give this a shot!!! Let me first say that, with any luck, someone who understands these sorts of terms and is accustomed to comparing in this way might chime in—RESCUE ME, FOLKS!—If they do, listen to them more than me!!!
Here goes:
Taste: The hub considers the white velvet cake his favorite “angel food” cake (although the white velvet has no angel food “tang,” of course, or rubbery stickiness that AF has). When he wants angel food cake, I know he really wants white velvet. The white velvet always reminds me of sweet milk bread. Sort of a non-eggy custard, if you can imagine that. The downy yellow is more like pound cake in its flavor. Sweet, buttery and, well, yellow-cakey.
Moistness: They are both very moist, but in different ways. The white velvet is a denser, compacter sort of moist. The downy yellow is a fluffier, lighter moist.
Crumb: They both have a nice fine crumb. I think the downy yellow is a bit air-y-er.
I’ve never tasted these side-by-side, mind you. They are both magnificent plain as well as frosted. My very inexperienced opinion is if you use are using a really billowy froofy frosting with a whipped cream lightness, you might prefer the downy yellow, as it might match-up better. If you’re using something with the weight of a buttercream, you could use either. If it’s the weight of a non-whipped ganache, maybe the white velvet over downy yellow.
Julie! Someone! Please help these people with an answer that is clearer (and maybe more accurate) than mine!!
Anyway, on the whole, I don’t think you could go wrong if you tossed a coin and went with the results! Good luck, and if possible, please post photos!!!