Depends on the subject. In most subjects, though, if you work hard enough, it's easy to get at least a 5. The main jump is from a 6 to a 7. I'd recommend A levels as well, though. Think over it carefully before you decide.
I agree with the above poster. I honestly think that a 5 in IB is easier than a B at A Level, judging from A Levellers I perceive to be smart getting C's or D's. It's jumping up to a 6 that seems to be difficult in the essay subjects. Then there's an even bigger jump up to a 7. I think this is justified by the rather steep drop off from 30-38 IB points across the world. (I can't remember where I found it, anyone know what I'm talking about?)