a) Thornton is a flat out more explosive scorer than IT. Not even a question. More explosive scorer than just about anybody really, but so hard to contain in a system.
b) the advantage, such as is it, with Thornton is that he doesn't bring the ball up. Oh my, what a chucker when he does touch it. But he can only chuck what he touches, and so you can control it to some degree (and indeed we have enough selfish players that we often do "control" it by simply ignoring him and having other guys chuck instead). That's its own problem of course, as you have to run a 4 man passing offense for fear that everytime MT touches it a shot could go up almost instantly. At the beginning of last year, his chucking disease was a primary factor in the offense stalling.
c) the huge issue with IT is that he is the "point guard", so he brings it up, and because he has a wide selfish streak, he just won't give it up. You use him to initiate a play, and he is running the play for himself. He is NOT as selfish/chucker as MT -- few are. But because of his role and ball dominance, the impact of his selfishness is just as much, maybe even more. Being irrepressible can be a good thing or a bad one depending on circumstances.
d) raw ast/gm is a deceptive way to look at things, but infortunately in this case ast/per 48 doesn't help matters much, as he's 38th there, sandwiched in between Paul Pierce and Dwayne Wade. My own little method of tracking the problem has to been to compare shot attempts by the PG to Ast. And there its even worse. Here is a table of some major gunning PG's and their shots/per ast:
FGAs per Ast
Irving 3.23
IT 2.73
Kemba 2.70
Curry 2.63
Barea 2.60
Westbrook 2.50
Lillard 2.42
Nate 2.41
Jennings 2.34
Parker 2.03
Lawson 1.96
Holiday 1.95
Jameer 1.91
Collison 1.77
He simply shoots much much too much, and passes much too little. He guns more than Russel bleeping Westbrook. More than Kemba. More than even his big brother Nate. That's not something you want or need out of a starting guard supposedly running an offense featuring Cousins and Reke. Somebody has to tame the shot happiness or he's not going to end up being part of a winning franchise.