I've been using a man/press combe defense at D3 and I'm not having the kind of success that I would have liked so I'm thinking of changing to either just man2man or just press.  Thoughts on whether you guys think it's better using one defense over the combo and if so, which to choose?

At D3 I would not use combo. There are two main drawbacks:

-At that level, you should be recruiting guys who have a lot of growth potential. Having to devote that many minutes to team practice takes away from their individual growth.
-It takes away from iq growth in offense and whatever your main defensive set would be.

Imho it's very little extra benefit and some significant drawbacks. At a BCS school it would make a lot more sense because there's a lot less skill development to worry about.

I agree. I would rather get my players to an A/A- in one than a B/B+ in two. Early on with potential when about every player capped out, I ran a combo with my DIII teams. Since WiS adjusted the high-high potential to allow more growth, it made less sense to run a combo.

I am leaning more towards M2M than the press, but I think both defenses are good. I run M2M with my teams in Naismith (switched from press after NC) and Knight and press with my team in Wooden (Washington (MO)). I usually feel more confident heading into the NT with my press team, but that may be more due to the qualities of my teams. I have had better NT runs with the press.