heat ammo. seems much more useful later on when you'll be fighting mostly machines on most missions compared to a skill that only activates when you get the chance to get a 2nd reaction shot and the first one was a success
H.E.A.T. Ammo. IIRC the damage buff applies to Rocket Launchers as well as the primary weapon, which is very useful for Cyberdisks. Sectopods for some reason seem to no-sell rockets, but a close-up Heavy (preferably with Metal Storm) melts them. Since both Disks and especially Pods are high-danger enemies, having a Heavy as an effective counter is valuable.
On the other hand, for Rapid Reaction Heavies aren't very accurate so hitting once let alone twice with Overwatch is sketchy. This ability might work well with Holo Targeting+Suppression, but from my experience with Suppression targeted aliens rarely move to trigger the Overwatch effect.