CB750 - Heavy, wet sump, 4 cyl (so 4 carbs, pipes, etc...), fast but more top end than low end grunt, a little top heavy, very smooth through the REV range, massive aftermarket, somewhat boring, its a Honda so change the iol and dont ride it like its a MotoGP bike and it'll last long enough for you to give it to your grandkids in exchange for them changing your depends at the home...
XS650 - Not as heavy, twin so intrinsically simpler to work on and service than the CB, fast but with more bottom end pull and less fire at the top of the REV range than an I4, somewhat top heavy, shakes more than a epileptic at a Pink Floyd show, lots of aftermarket, sounds 100x better than any inline four (street bike) when hopped up, its still a Japanese four stroke bike from the 70s so boring in stock trim...
All my opinions, except for whats fact. You decide which is which... Haha.
Both great bikes in there own ways. For me... Its a vertical twin (ie XS650, CB500/450, Triumph T120, etc.) Or a Vtwin (Ironhead SPortster, early Virago, etc...) for a mid size 4 stroke vintage any day though.