A c in economics is worth more than a grade a in media let alone a b imo, but if you just need some ucas points obviously a b is worth more ucas points, so it kinda depends on the situation.
B in Media, unless you want to apply to top unis - but as those don't offer "Business and Computing", and would anyway have higher grade requirements, I'd take the B over a C.
Provided that your other two subjects are strong subjects.