Not very helpful but the answer is....it depends. You will obviously have to put some money aside since neither option is particularly cheap. Assuming you can afford either, your choice boils down to whether you are good at studying on your own or whether you require more of a motivational atmosphere. If you can happily study on your own and have the self-discipline to see it through I would recommend somewhere like Bristol. If, on the other hand, you found that studying was not your strong-point at school you might fare better at a full-time school like Oxford. Self-study will be cheaper but won't work for everyone.
As a matter of interest, how do you propose to pay for your training?