First is first. Some fights were more difficult on 10, some on 25, so overall it balances out. First is first when it comes to heroic modes, 10 or 25.
Also if there was a 10 man guild getting world firsts back when Paragon was raiding 25 mans, I'd still be saying first is first and congratulating the 10 man guild. But there wasn't, so it's a moot point.