OK, to liven up the conversation, these are the considerations as I understand them, please feel free to correct or educate me, and to add to the discussion.
Weight: There are two schools of thought...light is better, heavier is better. Light better because its easier to move and transition. Heavier better because better to help control recoil to keep the dot steadier. I can see the advantages of both, but there must be one answer that is better for most situations. I feel that the guns generally are pretty heavy, and that the recoil is relatively low. That's why when I hear about people saying how the Sig MPX is SO much better than a JP because the recoil is so much lower, I have trouble thinking that there's a measurable difference. I've never shot an MPX so I don't know, but I feel that my JP recoils so little compared to pistol, that it's really not a big deal. I'm fairly strong, so running around with a heavy JP isn't a really big deal, and I can swing it around pretty well. I know...it's really about the shooter, not the tools, but if you have a choice to buy tools, you may as well buy the best tools you can. If you're going to add weight or not care about weight anywhere on the gun, the end of the muzzle would be the place to have it, because it would take more force to move the end of the muzzle. Kind of like the frame weights for Glocks near the end of the barrel. Aluminum might not hold up very well, but is cheap. It might also be very hard to clean because it is soft. Titanium would be the best material if you were trying to save weight, and Stainless would be the best material if you wanted to keep weight at the end. Because recoil is not a big deal, titanium would be the best material. But maybe I'm wrong on that and stainless is better, and don't worry about the added weight.
Comp design: I think it's amazing how there are so many different designs in respect to where the gas ports are. Some are on the side, some are on the top...You'd think that there should be one best way to direct the gas. At least in general. I would think that in regard to recoil control, with the fact that 9 minor doesn't make much, it makes sense to direct as much of it as possible upwards, or up and a little to one side (can't remember which side AR9s generally recoil, I think it's right). It's just frustrating that there are so many different designs, with everybody saying theirs is the best, when clearly one is the best and the rest are probably pretty good or just plain bad and look cool. So "tactical" or beautiful. That makes me think that maybe something like American Precision Arm's "The answer" comp may be the best solution, so that you could "tune" the comp to your particular setup.
Stir the pot.