Having worked across several universities and psychology settings, in the world of Psychology RHUL is generally regarded as having a better reputation than Loughborough. It also has it's own DClinPsy course, and although the two are quite independent, you are more likely to meet trainees and get to know people if you are highly motivated, whereas you won't get this with Loughborough.
That said, at undergraduate level both are acredited by the BPS so will be comparable. What you may want to think about is the type of research conducted, as these researchers are likely to teach some of the optional modules and it's nice to have your interests mapped onto those at the university.
Also its important you will enjoy your time there. RHUL is quite a campus bubble and that suits some people but not others. Loughborough has it's own pluses and minuses (nice market town nearby, close to Notts, but further away from the bright lights of London). It's always best to pick somewhere that suits you and you feel you will be happy at.