Marvel has always been better than DC when it came to dealing with diversity in comics, however that gap is nowhere near what the revisionists would have you believe. For instance, at first the X-Men dealt with no more discrimination than any other characters that weren't named the Fantastic Four.
While Marvel did have Powerman as that first black headliner, I'd have to say that making the primary Green Lantern a black guy is a bigger move.

They're really wasn't much of a difference when it came to how either company handled race. You'd either have 1) it ignored 2) the most multi-cultural gangs ever or 3) racism as shown by a black guy hating white people (which is still being done today "hi Nighthawk").