Personally I got a big soft spot for e-white so I’d lean more towards white for myself. Although I do have to say they are both pretty safe colors that go with just about any key set, as black & white are both neutral enough IMO to not clash with other colors. That said I will admit black is the more neutral of the two. Because I’ve put some sets on my white boards & felt it just didn’t look right. While I can’t ever recall having that thought with any set I’ve ever put on a black board.
Edit: I forgot to mention PC boards which are hands the most neutral, can’t really clash with translucent. So I agree with everyone saying PC is probably the most match anything case material/color. I guess we should also throw clear & frosted acrylic on that list too if we’re gonna talk up PC for matching any colors.