The recent they ask you to decide now is so that they can get you into the system and organise your 12 week scan. If you don't know much about them, I'd go for the closest one for the time being and, in the meantime, do some research and see if you can arrange a hospital tour for each of them and then, if you prefer a different one, you should be able to change it. After all, if you were to move to a different area (as I did), then you'd have to change hospital.
I ended up giving birth at Stoke Mandeville 16 months ago and would definitely go back there if I were to have a second. Crucially, the care during the labour itself was fantastic and I felt that they listened to me even when things got a bit fraught (DD's heart rate crashed). The delivery suite had just been opened so was pretty state of the art with en-suite bathrooms in each room. The after care was also excellent. A MW or a nursery nurse came to check my latch each time I fed DD, a student showed us how to bath DD (something I'd been getting really wound up about having never had much contact with newborns) and, fantastically, when I couldn't get DD to settle in the middle of the night one of the MW took her for a walk around the corridors until she dropped off. Having had an epidural, I couldn't get out of bed for a while after DD was born and DP had gone home so they had to do everything for me - get me water, pass me DD etc. They were obviously stretched and sometimes you had to ask for something a couple of times but generally I was really impressed and would go back again. A couple of things to bear in mind is that just before a shift change, the MW are less helpful (but then anyone is just before they go home) and, obviously, if you've asked for your water to be topped up but someone else has suddenly started to bleed, they're going to deal wit that rather than your water and, by the time they've dealt with that, they may have completely forgotten about your request.
If you're considering using the birthing centre, its in the same building and on the same floor as the delivery suite so, if there are any complications, its not far to go.
On a practical point, there never seemed to be any parking spaces and do take your own towels as the ones they have are about the size of a tea towel.
Friends of mine have also told me that the medical care at JR is fantastic and, of course, it is a teaching hospital. They weren't very impressed with the post-natal care but both have said that they would go back there if they had a second as they were so impressed with the care during labour itself which is what really matters.