That's a tough one! I have 10 of each and they have very similar personalities. They're all very friendly with people. A little pushy/demanding when they think you have something they want. They can be a bit aggressive with each other. Nothing crazy, but enough to notice. Actually, 5 of our reds are really nice and the other 5 are a bit obnoxious. All 10 black hens are still really sweet.
If I have to pick, I'll go with the black sex links.