I'm no expert in R-value but what I understand is, obviously, the higher the
R number the better it insulates. Therefore, I would assume the product with
the extra 2 R-value is a slightly better insulator.

Her are some sites I thought were fairly helpful:

http://www.roofhelp.com/Rvalue.htm
http://www.ornl.gov/sci/roofs+walls/...on/ins_16.html
http://www.powerhousetv.com/stellent...gs_000569.hcsp