Everything I have read says the Marlin has a good enough frame. Why wouldn't upgrading in the future be worthwhile?
BTW I am leaning toward the Trek. After going back to the shops, I really like the people at the Trek shop. Also has the benefit of 3 locations in town. 1 which is close to my work, and the other close to my home.
The specailzed dealer was a little out of the way and not as "beginner friendly"