Maine vs. Colorado advice

Hey gang - I've got some frequent flyer miles burning a hole on my pocket and looking for some destinations for this fall as the weather gets cooler (late Sept/early October). I'd likely be traveling alone. Probably four nights. Most of my hiking/camping experience has been around the Southeastern US (GA/TN), so the Colorado Rockies would be new territory for me, elevation-wise. Looking for a challenge (but not insanely strenuous), would like to avoid crowds, but ideally somewhat intuitively navigable trails/routes/sites. Would be flying in to Portland, ME or Denver, CO. Any advocates for one over the other?