some things depend on where you are also. Here in the northeast insulation and vapor barrier are almost essential. Steel reinforced still gets my vote. Make sure they pull the steel grid UP when they pour otherwise it easily gets pushed to the bottom and does not reinforce.
If you can afford it, go with 6" and definitely let the designer know that you will have tanks and possibly heavy equipment on it. They should be able to give you a psf (lbs/sq ft) load rating. It's good info to have for future expansion.
I used a surface hardener on mine. It reduces dusting and develops a shine on the surface of the concrete over time and use. If you may drag a pallet or something, it will hold up better but it doesn't strengthen the floor.
Make sure they don't pour it "loose". They like to water it down so that it flows quicker and easier but there is a range that is good. Depending on how big a pour, you may want to hire a concrete inspector. They will check slump, which determines how loose it is, and they can also check strength. They will fill a couple cylinders, then let them cure for 7, 14, and 28 days. At those times they will measure what the breaking force is. This is good insurance in the long run. If it fails, they did not meet the purchase requirements and you can have a leg to stand on if you are not happy. It's also pretty standard that they check the time the truck was loaded vs. the pour time. If it's too long, they send the truck back.
I've seen a lot of big contractors try to get away with stuff. The small guys typically don't since their reputation is personally on the line.