Well, I do Economics, and it's a good subject for you if you like current affairs/understanding what's going on in the news, because you see some of the things you're being taught in class actually happening in real life!
As for how well it combines with your other choices, I've taken similar subjects to the ones you're looking at (Maths, Physics, Chemistry), and I think it offers something different while still being vaguely related to Maths, so it puts a bit more variety on your timetable than Psychology does.
I don't really have much knowledge of Psychology other than that friends who are taking it say it's mostly taking/memorising notes, so if you like that sort of thing it could be a good fit.
At the end of the day, it comes down to what you think YOU will find most interesting and enjoyable - there's no point taking Economics just because some universities like it if you hate the subject!