what is the benefit of making it a sloping top tube?
Not much really. It is the fashionable thing these days though. I suppose in theory it might stiffen the BB and clearly makes stand-over less but overall I think it is an aesthetic thing.
does it “fit” or ride different?
ABG tends to go with t-shirt sizes on compacts since ST length can be handled with post length. I suppose someone might say compacts feels lighter while climbing out of the saddle due to lowering of weight but I really doubt it. I think the key in sizing the standard vs compact Vortex is going off TT length and understanding the compact’s “virtual” TT length.