Well, I'd say going all melee at 6th is more diversification for a Wizard than powering up, but any time a character gains something, power is involved. I'm not entirely sold on the idea that say Wpn Focus is more of a power up than Skill Focus; its all about getting better at what you want that character to do.
A character can go 6 in Fighter and not change much with 6 bonus feats; they get better at more stuff but its all the same schtick; hitting people harder and more often. He isn't going to be able to cast spells, and even if he devotes all his feats to skill bonuses, he's probably still not going to be a particularly good skill monkey. But if you go 6 in any of the other classes and then tack on 6 bonus feats, and its not that hard to horn in on the Fighter's action. (Even with 6 feats devoted to melee combat, a Wizard is still going to trail behind a basic 6th level fighter-- nevermind and Epic Fighter with 6 more feats-- but then he'll still have his 6 levels of Wizard to fall back on. I mean, just Fly and a few ranged combat feats would give him quite the edge.)
There are all sorts of other considerations that an actual game would bring into the discussion, things particular to the campaign's focus, setting and the characters. When you're playing a character or running a game you go by a lot more than just the raw mechanics, but I'm only focusing on the raw mechanics right now. I gotta push the envelope, Mav.