Hiya The Boosh,
Ahh its a tough one isn't it?! What subject are you doing at Exeter at the moment? I didn't dislike psych at Exeter, it was actually my favourite choice and I was pretty certain I was going to choose it until I visited Sussex and was really impressed by their department and realised how much more enthusiam they seemed to offer. Exeter was really friendly but Sussex put on a much more impressive show and gave much more information about the course. I guess I feel if I choose Sussex I'll be making an informed choice whereas I didn't really get much info from Exeter either way.
Back to your decision - bearing in mind you've already been at Exeter, I don't think doing an Msc at Sussex as opposed to Cardiff would lower your employability. Sussex obviously isn't the 'best' but I think its got a fairly good reputation for Psychology. I do think its important to go with the course, thats why I am leaning towards Sussex, even though I'd love to live in Devon. But saying that I do have my concerns about Sussex, I think it has a reputation as a loud, partying uni and I even read in the union's paper that students have been defacing library books there, which is slightly concerning!! Ah well good luck with choosing, its such a big decision and so confusing! I guess there are plus and minus points to every uni!