He's a student of the game, always asking questions and wanting to learn more from the coaches. Is the hardest worker on the team and is a gym rat (some worry he works out too much, in fact). Went from being a 16 year old playing midget who hung out with the wrong crowd and wasn't going anywhere, to being a 17 year old rookie on the Blades who was then drafted by the Montreal Canadiens. He's only improved exponentially since then, too.
Thrower is absolutely more skilled and has higher potential, but he doesn't have the same attitude as Dietz and isn't nearly as consistent.
I have to disagree with the fighting part, Dietz can be mean and nasty. He's got hammers for hands when he punches and he fought anyone in his first couple of years in the WHL. The coach has asked him to stay away from the fighting unless defending a teammate or the goalie. Dietz is the heavy shot on the power play and has more offensive upside than Thrower while being more fluid in all four directions than Thrower.
Thrower is scrappy but has to get a better attitude towards wanting to be a pro. His work ethic is questionable and he's let his head get too big after the draft. His skating needs a lot of work, if you watch closely he has trouble with his pivots which can be exploited by the pros, gets caught puck watching around his end and his angling is terrible. Thrower gives up the middle of the ice way too often and misses his hits where Dietz can crush people and plays more controlled. I could show you lots of highlites of the Blades being scored on with the two players involved at different times and you will see that Thrower is caught turning to his strong side only and putting his back to the on-rushing forward which is a bad habit and needs to be fixed now. In the NHL or AHL, the forward will see that he turns the wrong way just once and will simply get him to turn then pull back behind him once he turns his back. It makes it really easy to get to the slot for a scoring opportunity on Thrower with just a little deke and delay move. He needs to be taught how to box out better in front of his net and never give the middle of the ice, just let the player go wide and a bad angle then plaster him into the corner as he cuts to the net. He needs a lot of work on his non-plant leg turns so he's not weak on the pivots, it's in his footwork. I won't be surprised if the Canadiens return him to junior next year in order to see him be more dominant at that level instead of bringing him to the AHL. He will have to really improve from now to next year's training camp.
Just my opinion from watching him and Dietz for the last two seasons.
Dietz has progressively improve his overall game and uses his smarts to get himself out of sticky situations. As RS said, his work ethic will be his best skill with skating and IQ as 2nd/3rd. When you have those as your best attributes, the sky is the limit.
Thrower is more prone to inconsistent stretches, but his best skills are more physical skills that could make it easier for him to translate into the NHL (He's physical, great shot, and good frame).
As RS said Thrower has more potential, but Dietz is the safer pick. I do prefer Dietz as well, but because of his skating and his IQ in all three zones will do wonders for him, IMO.