I really haven't watched Stewart play...like, at all considering he's in the Western Conference, and he hasn't done anything to make him stand out when he did play the Rangers. However, looking at his stats, he has 202 points in 319 games. Callahan has 219 in 405 games. Stewart is a .63 PPG over his career, while Callahan is a .56.
Since 2009-2010, he's had a 64 point season (77 games), 53 points (62 games), 30 points (79 games), 36 points (48 games), and this year has 25 points (52 games). Not sure what the formula is for determining a players pace throughout the season, but he'll probably finish with over 50 points which is right around Callahan's average as well.
This guy seems to be a better player than Callahan when he wants to be. That's the huge turnoff though from what I'm reading. And, giving up a player of Callahan's stature (him being home grown, our captain, and giving it all) for a player like Stewart is what's worrying people. But, if he plays to his potential, he can be a very dangerous player.
I don't want to trade Callahan to be completely honest, but I understand he's not worth 6 million a year with 19 points on the year. He just hasn't been a factor offensively this year and while not his fault, does get injured.
I'm don't think I'm against trading Callahan for Stewart, but I think I'd want something else in the deal just because of how much Callahan has meant to this organization since he's been brought up, even if the value of their skills are pretty close.