It's only marginally better, in my opinion. The lesser of two evils. But, yes, in an RP setting, I would rather see a Mary Sue than someone screwing over Vader's characterization to make him into a dark wizard.
Though, in fanfiction... neither are good.
There is something that is worse than both (if only marginally): OP Mary Sues that are almost practically clones of an already existing character, but just a teensy bit different. I'd rather have someone messing up an existing character in an RP, because then everybody can yell at them for screwing up the character and possibly chase them out or pressure them into doing it right. When they make a blatant almost-copy of a character, you can't really tell them they're doing it wrong because they'll use the logic that, since it's not quite the existing character, it's their character and they don't have to fix it.
I just encountered such a thing in a Power Rangers-based rp I recently joined on a different site. Unique story, unique premise, mostly unique team half-made of self-inserts, but this new guy (newer than me) pretty much cloned a ranger that's already had his adventures, only arbitrarily rich and famous. Hell, he didn't even bother to change the guy's color or zord. I had just finished a rant there before I popped in and noticed the current topic in this thread. =w=