I need help with wheel selection

[ofnaman]

After mowing it over about ditching the 8 lug, I decided that's what I'm going to do. I will be putting in 5 on 5 bolt pattern axle's with 5" bolt circle. My question is simple: 15 x 6's front's with 15 x 10's rear, or 18 x 8's front and 18's or 20's x 10's rear's. I can get the 18's and 20's tire's for free (I am a tech at a Cadillac and Land Rover dealer and those owners throw away great tires), but I don't mind buying the 15's either (I can get them cheap). What do you think? Thanks