Well, for starters, Stewart actually isn't particularly physical. He didn't hit much in minor hockey or in the OHL and, presumably, that trend has continued in the pro's. Physically, he is much closer in style to Eric Daze than he is to Todd Bertuzzi. Although my observations of Bernier are limited, I would say Stewart is probably the more skilled player. (of all 3) Anthony is very, very skilled. Unfortunately, the guy is horribly inconsistent and seems to lack motivation. Which is a shame given his immense skill and physical stature. (the guy is an absolute beast, he just doesn't play like it) If he can put it all together he has the highest upside of the three. But I don't think he ever will.
In a bit of a peculiar situation, Tambellini - the smallest player - is actually the safest pick. He has the tremendous speed, is a smart all around player and has a blistering shot. Skillwise, he is the least talented of three. (with respect to creating their own offense, stickhandling etc.) That notwithstanding, if he finds himself with the right linemates, his speed/shot may enable to put up the biggest #'s.
Truthfully, I have seen the least of Bernier. But from what I gather, he has less innate ability than Stewart but appears to be a safer bet to reach the potential. (although still a riskier proposition than Tambellini)