Advantage: E-620.
The E-620 has a phase detection autofocus system, while the E-P1 has a contrast detection AF.
This results in the E-620 having a more responsive AF performance that's suited for quicker focus acquisition and lock.
Coupled with the availability of an optical viewfinder on the E-620 DSLR (none on the E-P1), this makes the E-620 the preferred equipment for such photography. Wikipedia states:
DSLRs are often preferred by professional still photographers because they allow an accurate preview of framing close to the moment of exposure …
Read more on the differences between phase and contrast detection AF, and on Olympus E-P1 vs Olympus E-620.