Are we talking a bowler who averages 3.5 economy, or hits 3.5 EVERY game?
Likewise with the batsman?
Because say you've got a great bowler, good econ, decent strike rate, then yes he is going to be a great asset to the team. However even when he's on form he's only going to bowl 20% of the overs, so if even 2 of the other 4 are having a mare then you can wave bye bye to the game. Likewise, if he's having a shocker, then you could be looking at 60-70 runs given away, unless you take off your prime bowler and use a part timer (which in honesty, most captains will take 59 bad balls from the prime bowler in the hope of that one peach)
As for the batsman, well, if he's a top order batsman, then he could well win the game single handidly if he racks up at fast paced ton, even if the rest of the team is stuttering (unless there batting like England!). If he's not firing, and gets out cheap, hopefully one of the other top 7 will get a decent knock to keep the game alive.
Personally whilst i believe both would be great to have in the team (hmm, taking two class players over one, thats a novel idea), i feel a star batsman can influence the game more than a star bowler can. Admittedly the bowler could do a Vaas and take 8 wickets, but the chances are remote in comparison to the big hitter running up 130+. Also a batsman can dominate an entire innings if he's REALLY on form, a bowler cant hit that sort of dominance unless he's paired with someone else who's nearly as capable, otherwise his hard work would be in vain