What is backspacing?
 

Which rims will stick out the most on my 4runner between the two? On a 15 x8 rim a back spacing of 3.75 or a back spacing of 3.25?

Let me help you help yourself. http://www.4crawler.com/Diesel/Tires.shtml#WHEELS Stock is 4.5" FWIW

3.25 will stick out further than 3.5.

http://www.jk-forum.com/uploaded/backspacing.jpg

As this diagram shows, the higher the number on the backspacing, the further outboard the wheel center is.

3.25" will poke out quite a bit.