Both are situational. Main benefit that I get from heavy is not heavy armor but ability to Rage Burst, it sadly doesn’t work with GL but does work on sniper rifle and hell cannon. Carrying extra clip is worth of being able to apha strike Chirons or Sirenas with just one of your guys - sniper with a Rage Burst. The other use is a double shot with GL using QA + BB for extra fun. Granted, Dash is more versatile in regards to positioning and getting out of trouble + return fire is absolutely neat on sniper. For this reason you can do a nice mix. Like two guys that can jetpak. One is H/S with GL and shotgun, which gives him 2 GL or 3 shotgun shots just from Quick Aim (rage burst doesn’t seam to fumble), can replace it with a heavy pistol to make sure he doesn’t fumble, but it’s a side arm anyway. Another is H/A with with the same weapons but ability to Dash into better position or MG/Hellcannon as these require good position to be effective. In both cases you get Rage Burst that super useful to kill something quick or damage whole bunch of enemies - try rage burst with hell cannon or shotgun into a crowd + you can add a Quick Aim on top of that for extra hilarity as it stacks and shotgun pellets can stay grouped for 200+ meters.
You can do reverse as well, as in S/H that is able to Rage Burst or just use hellcannon with all accuracy bonuses he gets from armor. I tend to see less and less effect from Return Fire on sniper if we are not fighting in confined area. Absents of Dash just ends to split squad into two groups, one is more mobile than the other but other has more firepower.