Ammo is obviously how much total damage it can repair. Isn't really much of a problem unless you're doing a very careless Sag A* (or further) run or have the incredible bad luck to run into a binary system at just the wrong angle. Once your ammo is gone, it can't repair anymore. Still, 3600 seems like a good enough amount to get most of your modules up from like 50% or more.
As to the other one, I think it's how much damage it can repair at the same time. e.g one repairs modules at a rate of 64hp/s, the other 70hp/s. So again, not that important unless you're literally 200% heat cooking yourself to death.
Going to be really intresting when the module changes come into effect and damaged powerplants provide less power, that's for sure.