I own pretty much every type of undershirt/pants with only a couple exceptions (bikini, t-back, tighty whiteys & the like).
What I wear/if I wear is dependent on the circumstances. For example, if it is hot out and I am to be in the heat any length of time, I go commando. You would, too, if you have ever gotten heat rash 'tween your nethers. If it is cold out, I don boxer briefs or boxers.
I almost always wear an undershirt. It actually ends up cooler, if it is directly against your skin to soak up sweat than a polo/T/button down by itself. Also, a wife-beater under a thick, dark T-shirt allows one to CCW IWB with comfort.
IMO, socks and an undershirt are much more important to comfort than underpants.