Assume your talking about Distance Learning?
I was up against the same decision. Had to go with OATs in the end for the simple reason that to be considered for their 'Waypoint' Modular Training programme, you have to do the ATPL Theory with them. Although I can see their point regarding completing all your training with the same FTO. It's silly really, because they allow Distance Learning, then why the
does it matter where I 'Distance Learn' I must €$£ ?
If it wasn't for the above I would've probably chosen BGS due to the praise they get (teaching style, content etc). I would weigh this slightly above having Oxford on my CV, because, after all, I would be on the Distance Learning course not full time classes.