I'd go for the supervisor. Your project might not work out and have to change. The people in your lab will change during the course of your PhD - you can't predict slight changes in lab culture. The situation regarding opportunities to teach, access to seminars etc. may change or turn out to be better or worse than you expect. But you may well have to interact with your supervisor every day. You're going to be in some way dependent on them for a really long time (how long do you think you'll need them for references, for example?) and they're not going to change.

Which do you feel more comfortable with? Do you think they'll both be equally good at supporting you with writing reports, your thesis, posters and presentations? Do you think they'll both take the time to look out for you - or be "on your side" - in terms of helping you get the most from your PhD and take advantage of the opportunities available? Do you think they'll both motivate and inspire you when you're having a tough time with your project? Is the person you think will be your supervisor the senior person you will interact with on a day to day basis?

Try and choose a supervisor you like: there will be times when they will frustrate and annoy you and it helps if you believe that they are basically a nice person. PhDs are tough enough anyway - you wouldn't want to be in a situation where you dread every meeting or email from your supervisor!

On the other hand if you have strong friendships/ a significant other in Oxford that will help keep you sane... that can add a lot of quality of life too.

Good advice here too: http://www.biology.duke.edu/johnsenlab/advice.html