Since I always use 2 or 3 AT-ATs, which deploy Army Troopers as their ability, and because the only use of infantry for me is to capture stuff, I don't build any at all. On some maps where AT-ATs are less useful (Endor, with the ewok swarm, for instance), you do need something else, but as Zardnaar said, you get plenty of stormtroopers in your starting forces (something like 3 platoons on every second planet), which for me is impossible to run out of, since, as I said, I only use them in tactical purposes. As for which of them is actually better in combat, I'd say stormies, but again, Imperials aren't supposed to rely on infantry anyway, with the choice of armor, you really don't need them that much.
Having said that, I'm yet to play as Zsinj or the Pentastar Alignment in 2.2, and they have AT-TEs now instead of AT-ATs, which means no free infantry. So I guess that changes things a bit for these two factions. I'd go for Stormtroopers over Army Troopers then. However, Zsinj has Raptor Troops, and PA has Storm Commandos and stuff, so there are different choices to make. All of them have their strengths and weaknesses, I'd assume.