I personally will only up a Tiger if I need HE rounds (town or hanger killing). The HE shells for the 17lbr are weak. (I will actually take a Panzer before a Sherman for HE work as well.)
If you don't need HE, the Sherman is better IMO. The green blends in almost perfectly with the trees, making it hard to spot if you are in defense. The gun vs. tanks is nearly identical to the Tiger's. The armor is worse, but I have been one-ping killed in a Tiger too many times to care -- I just assume the first hit will kill me anyway. The turret traverse is faster as has been pointed out. Finally, at about 1/5 the cost of a Tiger, it is a steal -- not that you will blow through those perks any time soon.