Botham was a better batsman, in my opinion. Imran was a useful batsman who contributed well and worked hard, but he was never a matchwinner with the bat, and never made many scores of really daunting size at test level. Botham has more runs, more centuries, and far more matchwinning performances with the bat at test level.
Imran was a better bowler, and by some distance when you consider his comparitive longevity. Botham was an awesome bowler early in his career but dropped off fairly badly, while Imran was consistent throughout and is one of the great fast bowlers in his own right, which you'd have to say Botham isn't.
Botham was obviously a far better fielder, and there's no real contest there.
As all-rounders, I'd say Botham was marginally better at his peak, simply because for a brief period he was almost unparalleled, but Imran obviously maintained his brilliance for far longer and it shows in his statistical record.
Overall I'd probably have Imran in my team first. He contributed well over the length of his career, was a good captain and a dedicated player, none of which you could say about Botham. However, Botham was the truer all-rounder at his peak, in the sense that he could turn a game with both bat and ball, or both, and did so on a fairly regular basis for a few years early in his career, and if it came down to that 79-82 sort of period of Botham's career I'd probably have him over just about anyone.