If you like red better, I'd say keep the red. At least, from my POV, color is very important and I really like black. I know it's boring, but as an adult, I've only ever bought black cars as I like it so much.
I'm not so big on rarity either, and I think you're right that the tan interior goes pretty good with the gold wheels.
If you're not as hung up on color as I am, the next thing I'd find important is the body, more specifically, body parts that you can't easily swap like quarter panels. Does one have previous body work on a quarter panel, body filler, etc. while the other has clean original quarters? Body fillers might not give you any trouble for many decades if done well, just a skim coat, and stored indoors. I just feel like there's a lot of value in a really straight car.
Other considerations, how much unobtainium do you still need for each car? Do you have the time to get the black one running, or will you get to it "someday" - but "someday" just never seems to get here (that's the story behind all my projects ).