I currently own a full (for my needs) Canon APS-C and Olympus MFT kit. I was recently on a 3-week trip in Central America and brought only my MFT kit as I was traveling light. I took 4,134 photos, shot on top of a volcano, several Mayan ruins, in rain and heat, and (informally) at my friend's wedding - and didn't miss my heavy and bulky APS-C kit at all. In fact, I have hardly touched my 550D after I bought Olympus E-PL1 earlier this year. Prints up to 13x19" look gorgeous as well. So now I'm considering selling the APS-C kit as redundant.
But I still want to have a digital camera for studio portraits. Digital in studio for me is mostly for getting lighting right (chimping), and backup to MF and LF film. MFT doesn't have the required shallow DOF, E-PL1's UI is quite fiddly for chimping, and it doesn't even have a remote release. Most importantly I can't imagine it playing well with external flashes, and mounting a 580EXII on it looks like a bad joke. I haven't tried it with external flashes, though.
I'm considering either keeping my 550D for studio, or moving to 5D (the original, used). The latter would offer DOF closer to MF/LF than 550D, and I already have an excellent portrait lens in the 135L. I still might need faster glass than my f/1.8 85mm for full-length portraits, but I'd need to buy one for facials anyway if I keep the 550D.
Any thoughts? Pros and cons I've missed?