I would go with UMD over GW...GW is a small hospital with limited variability of cases seen. If your in it for pure learning experience then I think at UMD you will see and learn more.
I go to GW and will be pursuing residency at UMD. I did a rotation at GW. The GW rotation is structured like this... you are assigned one section per week among CT, Bone, Chest, Neuro. You can do other sections, but you have to request them. We get didactics (approx qod) among those respective sections. There are 5-6 days where you have a hot seat session with Dr. Rapelyea, the course director for 1 hr. Those read out sessions are based on some of the didactics so you get to apply what you learned. During the bone week you are assigned a teaching file to go through by yourself or with a partner and then Dr. Brindle goes through them with you together. You also attend noon conf, and am conf is optional.
As far as the quote above from blaise77, I can't even begin to address the ignorance of his or her quote. Your goal as a medical student should be to come out of a radiology rotation being super efficient at CXR's, proficient at bone x-ray, AXR, have a general understanding to the approach to a body CT (understanding windows, organ approach, etc), and be able to pick up a bleed on a neuro scan. You'll have a final exam, partially computer based testing you on those skills. Zebra cases should not be attempted at our level... now with that said, you'll see tons of crazy stuff at GW.
With this all said, when I interviewed with Dr. Pugatch, he showed me many thank you cards from the students in his rotation. He is a hilarious guy and a very dedicated teacher (who I look forward to learning from). I don't know the structure of the rotation, but I'd gauge you'd learn a ton while you are there. UMD really has a phenomenal dept.
You can't go wrong either way.