Actually 1.4 to 1.7 is only one third of a stop faster. You'll get about a stop more DOF on the m4/3 body at the same focal length though.
For most purposes you'll be hard pressed to tell any difference in image quality. EOS will be better for focusing on moving subjects. The EPL-1 is more likely to travel with you and therefore used more often.
The field of view will be considerably wider on the E-PL1 w/ the 20mm than with the T1i w/ the 50mm. Using the standard 35mm equivalence*, the 20mm on :43: is a 40mm equivalent (40mm x 2.0CF), while the 50mm on the APS-C from Canon is an 80mm equivalent (50mm x 1.6CF). Another way to put it would be that the 20mm on the :43: will have the same FOV as a 25mm on the Canon.
* equivalence gives the focal length on a 35mm/full-frame camera that would yield the same FOV as a way to standardize across sensor sizes. It's not actually a longer focal length, but rather a smaller crop of the same image due to the smaller sensor that yields a similar FOV as a longer focal length would on the larger sensor.
The Canon EF 24mm f2.8 would be the closest equivalent on a T1i to the Panasonic 20mm f1.7 in price. The design virtually dates back to the start of the EF lens mount in the late eighties but it is still quite highly regarded and is a lot smaller and lighter than you's expect when you see it in the flesh. It is infinitely more useful on an APS-C sensor as a standard lens than the EF 50mm f1.4 or f1.8 which in fact become short telephoto portrait lenses. I don't know why either the EF 24mm f2.8 or the vastly more expensive EF 24mm f1.4L would be called macro as they are most definitely not macro lenses.