At 500 points I think you need to consider what you are playing against. Consider the number of shots you will need to take our a bunch of Nids or the bounce back phase thing that Necrons do.
Auto Cannons are good, but if you are going to run an Armored Fist unit I like the Las Cannon option for a heavy weapons crew.
I also usually have one of the 3 choices in my Armored Fist squads, Melta gun, plasma gun or flamer. Gernade Launchers are not any good against vehicles, but will drive a nids player crazy if you can hit. Other than that go with the other choices.
Chimera weapons can come into play as well, Multi Laser is always a good choice. Some people like the Auto cannon version if you can get the parts. I like the hull mounted Heavy bolter as well.
With a Armored fist unit the number of shots is important.
When you move up to 1000, you can still use one of these as a troop choice, but the other needs to be a platoon HQ with two squads.
If you want elite units go with Veterans as they get an addition assult weapon (3 instead of 2). Storm troopers are good, but I like them for infiltrate or deep strike units.
Good luck.
Warrior 50