Nationally, at least as far as AROTC is concerned, there is no difference between a SMC and a normal college or university. All SMCs do their commissioning through ROTC just like other state and private schools. I believe that slots for special training (Airborne, Air Assault, etc.) are determined based on detachment size as well as national availability, but I could be wrong. It also depends on Detachment Commander approval - all of the winter slots available to us this year would have meant missing the entire first week of classes, and the Colonel said academics come first.
As far as branch selection, cadets from SMCs are included in the same national Order of Merit List as ROTC cadets from around the nation. We also go to LDAC between junior and senior year just like our counterparts at normal schools. As Bruno alluded to earlier, your connections with Norwich may help you down the road if you make the Army a career, but only through the Alumni connection. In my opinion, perhaps the most valuable thing you can get from that connection would be a mentor.
If you haven't already done so, do everything in your power to visit all the schools you're even remotely interested in. There's only so much you can glean from websites, catalogs and phone calls. Seriously. VMI was #2 on my list of potential colleges 'til I visited; in the end, I didn't apply to any other school!
Also, one of your primary considerations should be the overall education. Academics cannot be overstated in this consideration, but you also have to think about the intangibles - the lessons that won't show up on your diploma or transcript. Do you want the physical and mental challenges of a SMC, or do you want practice juggling the part-time military/part-time civilian lifestyle?
I don't know anyone I graduated high school with who have accomplished some of the physical challenges I have as a normal part of my college education, but something I think we do miss out on is learning how to turn the military off at the end of the day. It is our life! We're never not in uniform (unless we're leaving for a furlough), and we are quite restricted when it comes to being able to leave for the weekend, or even leave campus in the evening. Again, this is based on my experience at VMI; it's best to talk to cadets at Norwich to find out the particulars of their structure. Better yet, talk to Rooks to find out what's up. This isn't even scratchin' the surface of the tip of the iceberg. There are hundreds of pros and cons to both the military and the civilian educational systems.
Both will teach you time management. A SMC will teach you time management by putting you in a structured environment set up specifically to give you too much to do in too little time, so you'll learn how to prioritize and say "no" to the good in order to say "yes" to the better (ie, graduation.

) A civilian school will provide you a heck of a lot more freedom, but with that freedom comes a lot more temptation to waste time.
Again, on academics, just because both schools offer degrees in Political Science doesn't mean that both programs were created equally. I may have gotten a more prestigious degree at an Ivy League, but very few other colleges provide the opportunity to work one-on-one with professors that VMI does. I can't count the number of Ph.Ds I've worked with directly, because the small school atmosphere means no grad school students will be filling in while the big important Ph.D is off researching.
This is the kind of thing you want to ask about. I would think that Norwich would have a similarly close-knit group, where faculty chooses to teach there in order to actually
teach and work with their students. When I had surgery in October, three professors (all with Ph.Ds) opened their homes to be in case I needed a more sanitary place than barracks to heal up, or in case I just wanted a break.
One thing any SMC will teach you is how to fail. It sounds strange, but this is an uncommon skill in our society where teachers grade papers in green ink since red is "too humiliating", and where everyone gets a trophy just for showin' up. If you've been successful enough to earn a four year AROTC scholarship, this is something you should definitely consider. At any military school, you're bound to fail at something at least once every day. You'll either get sick of it and transfer out, or learn how to dig deep and overcome the constant failure. Oh, and just as a head's up, it doesn't end after freshman year.

(As a rat, I went to a statistics peer tutoring session and the tutor, a First Classman, told us that VMI's new recruiting slogan ought to be, "VMI: Where Your Best Is Never Good Enough Since 1839!")
Let me pass along to you the most sound piece of advice I ever received as I was making my college decision: the academic institution at the top of your degree has nothing to do with what kind of officer you will be. Whether you go to a SMC or a regular college, you will be equipped with skills that will provide a strong foundation to be built upon, so that one day you may
become a good officer. Good officership takes time, intentionality and a willingness to learn; it isn't granted based on where you happened to spend a few years of your life.
Good luck! Hope this helps.
Jackie M. Briski
VMI Class of 2009
First Class Private