I'd like to mention the reason for the DM6 is portable observing. My experience is that a manageable 5" refractor is a lot easier to set up than a behemoth 7"
The larger longer refractors require taller tripods. The TEC website used to say something to the effect that the 160 was a small increase in aperture but a large increase in cost + weight.
Also note the popularity of the AP 130, TSA 120, and TEC 140. These scopes are in the ideal range of size / length and offer great optics. I believe this is one reason for their enduring popularity.
My point is don't get wrapped up in aperture fever to a point that negates the portability of the DM6.
On on the other hand,
Since the OP is young and strong , I suggest he try every scope possible and form his own conclusions,
A reflector like in post 22 above, the eyepiece is too high off the ground for seated observing. I have an 8" f/4 Newt that ideally requires a half-height Losmandy tripod.