If you have the lenses form your other canon, I would go with the 550D. Less of an investment. And Panny is very limited on it's lenses still. It can only use panny and olympus lenses right now with AF in M4/3. And only olympus lenses with 4/3 with AF.
I will be getting a oly epl-1 in april, and I find that even my 500D is a bit big for some of my travel needs. M4/3 has it's place. But I know it's limits, so it will only augment my 500D.
Also if Auto Focus is not a concern, M4/3 has over 600 MF lens option with the different adapters out on the market.
Super Frequent Flyer, no joke. Ex Patriot and loving it.
Canon Eos 60D, T1i/500D, Eos1, Eos 630, Olympus EPL-1, and a part time Pentax K-X shooter.