Just my opinion based on late last year's observation, plus what I've heard from scouts...but count me in the "Towle's defense is mediocre" camp.
I guess is all depends on what you consider important for catcher defense. From all I have heard about him (having not seen him anymore than most of you), he is very good at calling a game (or at least projects to be good at that), he works well with pitchers and his blocking is just above average (with signs of improvement) but that his throwing is average at best. I have heard multiple reasons for this (as the scouts don't seem to agree which is the reason for it).
So it seems like he projects to be very similar to Ausmus in defense, although Brad has refined his non-throwing skills much much more, so Towles needs to learn Brad's dedication. From what I have heard, Towles does seem to want to improve, so that is also a good sign.
People need to realize that Towles has really good potential, but he is making some serious jumps in levels that might result in some learning curve issues during this year. But I think the hope is that with Brad there to mentor him, hopefully he can help Towles adjust more quickly. Much will depend on Towles, and specifically his attitude this whole season. If he shows the attitude he showed in 07, things will go great for him... if he shows the attitude he showed in 06... there might be some real rough spots.
As for his hitting, I don't care how much "pop" he has as long as he is not counted on for that. If you are not hitting in a "power" spot, I would be willing to sacrifice power for BA/contact. But as a catcher the first priority should always be about his defense. If he does that well, I will accept whatever offense he can provide.