Excluding about the first 18 months or so of this HD generation (The first 18 months for the 360 and the first 6 months or so on the market for the PS3), I'd say 90% or more of the classic games that have been ported to one platform since then has also been released on the other. You'd be hard pressed to find any classic games on Xbox Live Arcade or classic games on the PSN alternative of XBLA that are exclusives since 2006.
Can hardly fault Microsoft about any PSP Minis. They're PSP software that just happens to be able to be played via a PSP emulator on the PS3. And we're not talking about things like Game Room for the 360 or Sony's rerelease program for PSOne, NeoGeo, and TG16 games. We're talking specifically about Xbox Live Arcade and Sony's equivalent.