The larger XC batteries have more available power for the tool. If you put an HO battery on an M18 impact wrench, it sounds like it found a new gear. I thought what the tool was rated for, is what you got on any battery, but it is clearly not the same.
First, both are 1st generation. Nothing has been replaced. Fuel line is not a replacement or a new generation for a non-fuel tool.
All Fuel tools are all brushless (though not all brushless tools are Fuel in their lineup, but very few), and that would indicate the newer model.
I'd go with Fuel 3/8" Stubby, and probably the ring version. Unless you are working elevated, or really prone to dropping sockets, the ring is preferred. The ring is also much more available (in store at HD, but no detent model), and more sales/discounts on it.
The other option if you do get a detent model, is to file the detent down so it doesn't have so much holding power.