Renting a Wheelchair/Scooter - Which One? [Question]

I posted recently about how I was going on vacation and needed to know how accessible my destination was before I really decided on a wheelchair/scooter or not. Well, I took an outing yesterday and discovered I really will need a wheelchair/scooter in order to be fully mobile - it's not an option anymore, it's a requirement.

Problem is, I have no idea where to start. I've always walked, even when I've struggled to do so. What are the pros/cons of a manual wheelchair versus a power wheelchair versus a scooter? As far as I can tell, the main benefit of a manual chair is that it's more compact and easier to pack, but will it help me if I don't really have upper-body strength to speak of?

If it makes any difference, the destination is Washington DC and we'll be using both car and subway to navigate. We'll likely be doing the usual touristy stuff on the National Mall.