Mag shank lug nuts are usually used for hub-centric wheels. Acorn-style nuts are typically used for lug-centric wheels. Unless you have wheels that are hub-centric, I would stick to acorn nuts. Also, I'm curious why you want to use 15" wheels instead of 16". The tire selection for 15" wheels is quite a bit more limited on 15's. It seems that many newer AT tires are only available for wheels as small as 16 and/or 17, but not 15.