Except for Stony Creek there's not much off road riding to be had in most of the Metroparks. There you'll find 12-15 miles of MTB trails and only the most hardcore (read: crazy!) would want to ride a 'cross bike on those trails.
Kensington is connected to nearby Island Lake Recreation Area by a paved link and there you'll find milder MTB trails somewhat suitable to 'cross if you got the skills.
That said I see you are in the downriver area and therefore would probably be riding mostly Willow Springs right? Not much offroading to be had there that I know of.
I'm sure you will find little bits of 2 track here and there but the main feature of the big MetroParks are their extensive network of paved paths.
As far as the choice between the "fitness" bike or the cross bike I'd say stop in to your nearest friendly Trek dealer and get some saddle time of both of them.