This is going to sound retorical, but figure it out for yourself. You also first have to get into school, I would be more worried about that more than anything. Take a tour, ask some students, get a feel for the area. The arguements about this is better than that is all a bunch of hear-say. Is there any hard literature that says U of M is better? NO. So don't listen to what old farts who graduated from school in the 70's and people on this board who often talk about issues like "the best residencies." Nobody really knows! I go to U of M, I don't even know where Mercy is located now that they moved. I should have an opinion, but I honestly have no clue about the other school. No other student honestly knows either.
The cold hard truth about Dental school is it is what you make of it! If you want to be a better clinician then work more cases, work with different faculty, do different proceedures, take additional courses, you get the picture. Picking one school over the other is in no way going to affect your specializing oportunities. Whoever told you that is full of crap. When it comes to stuff like this it's just like playing a game of poker, you gotta have the nuts no matter what. If you really want to do something set a plan and execute. Tons of people come into school wanting to be orthos and make $500,000 doing nothing. The reality is most people are crying by Christmas time when they realize they've already put themselves out of contention.
So get accepted, make a gut decision based on what you desire (for me I was more concerned about which schools were within walking distance to Taco Bell), and if you want to be a better clinician, get higher board scores, or specialize (It's alot more difficult than you think, not trying to discourage you, but giving you some objective perspective) then work harder to obtain those goals. The ball is in your court. Good luck kid!