Best IQ: E-PL1 or Canon 550D

It depends on the usage , the image quality at day time with good lenses can be very similar. what the EPL1 is incapable of doing is fast focusing and in order to be able to see in the sun you would need the viewfinder. Furthermore ORF raw files are very strange and badly implemented in most raw treatment software. But again I do have a Pen EP1 which has a very similar electronics to the EPL1 shooting in Jpeg with a little WB correction gives great results. In comparison to my Pentax K-M it stands the comparison and in fact it gives less noise in low light condition. In conclusion, if you need a fast and responsive reflex go for the 550d , i can also recommend you the pentax K-X , and the alpha 450.

On the other hand if you need a pocket camera , that will give you great results and fast autofocus is not your priority, then the pen EPL1 is for you.

 

IQ capabilities of both will be very similar - and this probably goes for a huge number of cameras today (with sensors less than 'full frame')

this active thread covers things well - https://www.mu-43.com/f43/g1-vs-dslr-image-quality-4187/

usability and the appropriateness of the AF system for your needs will vary

'dof control' will be a function of aperture and magnification (framing) to the sensor - without getting into a debate on equivalence these will also be broadly similar in principle but the Pen will permit the use of quite economical legacy glass in MF mode with an adapter.