Which is better for an Alliance Infiltration Unit?

Reegar for credit farming. A lot people are against it, but its the most efficient way for farming.

Claymore and only Claymoar. Cloak, Boom! Snap Freeze instantly will reload cancel for you, boom again.

Fists.

"Better" is every shotgun,best-never play it.

There's your answer right there. As has been pointed out weight of weapons on Infiltrators is largely irrelevant as long as you never (or very rarely) use Cloak to hide, and the AIU's heavy melee often gives you excellent opportunities to cancel the reload animation of the Claymore. That said, I would probably try to learn the true glory of the High Lord by starting out with a Krogan Sentinel, QFE, QFI or Human Soldier.

Other than the Claymore I would use the Crusader, Raider, Venom, Graal, Wraith, Piranha, GPS, Disciple or Eviscerator, probably in that order of preference. I basically only need an Omniblade holder as I run a full melee bot, and the only lube comes in the form of Berzerker Gear. You'll note no mention of the Reegar in that list. It's a gun which has polarized opinion on BSN for 6 years, and one that I personally despise. It's immensely powerful, especially if used on PC and doubly especially if done so with Incendiary ammo equipped. But it's not a Shotgun. It doesn't go BOOM! I have zero interest in score in a co-op game and I'm not the kind of person to take any pleasure from having to pretend I'm squirting a jet of rattly toxic piss which magically melts enemies, especially when there are 10 other options which are all more fun. Hell, I'd rather use the Scimitar, so make it 11.

On a point of order, the Piranha is good against armour while the Reegar is only good (epic) against Shields and Barriers. The Reegar must have AP/Drill ammo and/or heavy piercing to be effective against armour. My honest tip is to save it for solos. 

Snapfreezeless phasic Crusader; if you can land headshots consistently. Imho Crusader is one of, if not the best/strongest gun in ME3. Did I say I love Crusader? Yeah. Crusader. Mmmmm...

Anyway, seeing Reegar AIUs in lobby makes me always wanna switch to Crusader AIU; by the time they get to target, all of them are headless. Just don't pug with it random gold, it's almost disrespectful.