I've done both races in the past and they are both really fun. For a quick summary:
VA Beach
Pros:
1) Finishing on the beach beach is really fun. It is so nice to jump into the ocean after the race.
2) The free concerts Friday, Saturday and Sunday nights are excellent. Easily the most fun part of the weekend
3) Just being on the beach during labor day weekend is cool. Sure VA beach is a little seedy, but it is also fun.
Cons:
1) Weather can be really hot and humid
2) Course is nothing to write home about. I actually didn't like it that much
3) You have to bus out to the start
4) Hotels are very very expensive due to labor day weekend
PDR
Pros:
1) Everything is right in downtown philly. The expo, the start and the finish
2) Hotels are cheaper, lots of variety of places to stay
3) Rocky Steps!
4) Course is faster and better than VA beach.
Cons:
1) No post race concert
2) VA beach is more of a whole weekend long event / party, the PDR is more of just a race, granted its a fun race, but it is still more of just a race.
Overall I would say that with the exception of doing the Goofy, that VA Beach was one of my favorite race trips ever. You'll have a great time.