Hooray for another Michigander!
I should be careful, though, with the blatantly obvious icon I have... the state's pretty well divided between Green and Blue don't hate me if you're an MSU fan
I started out on club rides with my city bike too... I actually rode it in a mountain race this fall! (interesting experience that I wouldn't necessarily recommend... unless you're willing to make lots of repairs afterward! )
It's a tough choice between mtn and road. I know a few people that adapt their mtn bikes for the road by getting skinny tires, and they keep up with the roadies pretty well (they def. move faster than i do!). Personally I think mtn bikes are more comfy (you're not as bent over), but even with adaptations, a roadie is def. the best for on the road! (go figure ). i have a road bike that i love (it's definitely lighter and faster than my city bike!), but it took some getting- used to at first.
are you in the LP, UP, "mitt", "palm"? I know there are some great trails up north, and there's the Iceman race every fall (HUGE mtn bike race near Traverse City) plus a ton of rails-to-trails (some unpaved, so you might be better off with a mtn bike). but then again further south there are a bunch of clubs and club rides that are predominantly road riders, and road riding is fun on the paved roads through farmland.
I would suggest trying out a few frames, test riding them, and seeing which is more comfortable for you. Talk to your LBS too, and see what they'd recommend for the type of riding you'll be doing. You could always look into used bikes, too. A lot of racers baby their bikes and sell them after only a year or two (as happened in the case of my "new" bike ) Then you might be able to get the best of both worlds!
Oh, and will you be riding through the ever-so-lovely winters we get? knobby tires come in handy!