I think you need to look at the chainrings and cassette to determine what the bike can do. The hybrids usually have 48/38/28 chaninrings and 12-32 cassettes. A mountain bike will usually have 42/32/22 chainrings and 12-34 cassette. That is a big difference for climbing on a trail surface and going slow over real rough trail. That 22/34 combination will keep you on the bike through some rough trail that would have you walking the hybrid.

If you are interested in more "off road" than pavement in your hybrid mix then I would suggest a hardtail mountain bike with lockout fork and slick center tires like the KendaKross or 24seven.

I really like the Kenda Kross tire. It is not on their website, but I find it at many bike shops. It has a smooth center area with knobs on the sides. Fully inflated to 65 lbs they ride only on the smooth part. I over-inflated mine to 75 regularly and they roll very nicely. For trail and back roads I'd deflate them to 40 or even 35( this is going to depend on your weight). Then the knobs on each side of the center make contact and they do very well even in soft surfaces.