I think you have your numbers backwards. I have never seen FG listed with a R value exceeding 3.5. Check here, for ex. http://www.allwallsystem.com/design/RValueTable.html
Rigid foam is far superior for 3 reasons: One, the R/inch. Secondly, when you look at the whole wall assembly, as typically installed, FG R's drop way off because it is impossible to install perfectly and it is inconsistent in its structure. Finally, FG batts are not dense enough to prevent internal convective loops, further eroding the advertised R value. Sure, you pay a little more for rigid foam. What is your goal, though, when insulating? To put in something cheap or something that will pay for itself time and time again? If you want batts, get mineral (rock) wool, cellulose, cotton, or wool. Read about insulation on buildingscience.com, greenbuildingadvisor.com, ORNL site, etc. FG batts are a marginal insulation, but not good, by any means. Dense packed FG is good stuff, but batts, basically, stink. I have 13" of batts in my walls (installed in 1980, as I did not know better then) and they are doing a pretty good job, partly because of how they were installed. However, I have no use for them anymore, as there are better products readily available, as noted above; I will be dense packing cellulose, which is far superior to FG batts in a few ways. So, that is the skinny on FG batts as I see it.