How much real difference is there in these 2 steels. I dont know anything about metalurgy. Does one last longer than the other?
Can someone with a machining background help me out here?

No big differance ,just get it chrome-lined...

How much difference is there between regular gas and plus grade? Or between plus grade and super? If the choice is before you, and the cost were the same, which would you get?

I wont argue the difference isnt that much, but no one can argue that 4150 isnt better either. So if you have a choice between the two, why wouldnt you get the best quality steel?

As Neil says, all other things being equal, might as well go with 4150.  
[begin whine]
If Bushmaster offered mid-length barrel assemblies and/or 1/7 twist rate (aside from their limited stock of Govt. profiles), this would be an easy decision.  
But they don't, so we're forced to look elsewhere and then weigh the trade-offs.
[/end whine]

I'm not faulting Bushmaster.  Their business decisions are no doubt complicated, given the current political landscape.

Hoping for the sunset...
Randy

To be honest, if what I wanted was a mid-length gas system, I would not let the fact that Armalite or Rock River uses 4140 steel keep me from it.