Barrel length will affect velocity, which will affect POI.
I'm slightly confused as to what you mean by "mid-length rifle". Are you talking about the gas system or barrel length? As you can get two barrels the same length with different length gas systems: like a 16" barrel can have either a mid-length or carbine length gas system. I've never heard of a "mid-length rifle" before though.
If you're asking about the gas system's effect on bullet trajectory then I don't think there is one worth mentioning.
As far as lighter/heavier ammo is concerned, factory ammo is loaded to certain specs and do not go over a certain pressure. Both rifles can handle the same ammo. Mid-length gas systems are more marketing (IMHO) than nessicity. If you're racing the clock to win a competition where .01 seconds matters, a softer recoil impulse may help. If you're hitting paper on the weekend for fun you could probably shoot them both side-by-side and not tell the difference.
Barrel twist really doesn't go by bullet weight. It goes by bullet length. Generally same weight bullets are the same length, and it's easier to say 77 grain bullets out of a 1:7 twist rather than .87" bullets out of a 1:7 twist. (no idea on how long a 77 grain bullet is actually, but I have some noslers and sierras at the house I can measure tomorrow). The formula to determine proper twist rate uses bullet length though, not weight.
