The research rating of the Psychology department at Hull is relatively poor (3aC) and almost as bad as Portsmouth (3aD), when compared the research rating for Bangor, which is outstanding (5A*, the highest possible mark).
I think the RAE is an excellent measure of how well a department is doing. It measures the quality of research produced by the department, and generally speaking, if department is producing excellent, world-leading research, then they are likely to have people working for them who are outstanding in terms of their subject area. This is obviously crucial when studying at degree level as these are going to be the people who are teaching you. I find a poor research rating extremely concerning, and if I was considering going to a department who had failed to achieve a high RAE score, I would want to know why. Obviously, teaching quality is extremely important too, so you should try to find out what both of these departments have scored on the TQA as well.
I've never visited Bangor, but I have been to Hull numerous times, and I have to say I don't find it an attractive place at all. However, you should bear in mind that I've not been to the university, I'm just speaking about the city and the surrounding areas (but bear in mind, you'll probably only be living on campus for your first year). If you are coming from Cambridge (where I am also from), I would be surprised if you didn't find Hull considerably less attractive and more run-down in comparison.
If I was in your position, I would choose Bangor, but obviously you need to weigh up all of the factors, decide which factors are most important to you, and make the choice that you feel most comfortable with.