I have both and the 15X8 with 4.500 back spacing is the hands down winner for clearance purposes. It pushes the tire in "just enough" versus the 15X8.5 with the 3.75 off set. Plus the 15X8 has the outside "look" of a 15X7 wheel . I wish they made a 15X10 that way.
'66 GT350H-3 time cover car - Car Craft July'77,Modified Mustangs and Fords Feb2011 ( w/article), Mustang Monthly June 2014. Bracket raced by me for the last 44 years. Yes it is a real one.
'68.5 R code GT fastback
'67 Ranchero 408W -427 glass hood
'78 F350 "Oleynik" car hauler
Ford Experimental parts collector