4140 spec steel, give or take allowable variations in alloy percentages, is all the same. You're allowed a slight variation within a given range in the various components like carbon and whatever else makes steel 4140. The last two numbers refer to carbon content. Without looking at actual ASME specs, that seems to be the only difference between 4140 and 4150.

NO AR builder/manufacturer makes all the parts. They're all pretty much buying from the same sources. It makes no supply chain reason to buy different quality stuff for the products coming out of your factory. We bought M&P15s as soon as they were available and they ran right along with Colt 6920s. In fact they seemed better than the Colts (Yeah, I know that's heresy in some quarters, but ya can't argue with demonstrated fact.)

This was in stark contrast to the performance of another "name" brand that obviously was using fire control parts-and bolts- that obviously had failed to meet specs in some manner. But, I expect the were both cheaper and available.

IIRC, the Sport lacks a heat shield in the handguard. Check the factory website for details between the selections.
ETA: check the factory website about receivers. Edge to the forged receiver over the billet.