Like others have mentioned, Nitrogen is used for racing because it does not expand as much when hot and tire psi can remain stable. That's really the only difference. 44 psi of Nitrogen looks the same to the road as 44 psi of air. Will be interesting to see what your experience is, but I'm betting you won't see a difference. Also, the difference (if there was any) would be undetectable over the noise caused by other things that affect MPG (weather for example). I'd put the $20 towards gas... :-)

On the topic of psi in general, you'll get lots of opinions. There are a lot of folks, including myself, that run high psi and have noticed a benefit (I run 44 psi - max is 51). A few here run at the sidewall max. I always backed off a bit from the max, but that's just me. With air, 10 degrees will change the psi by one.

*** Retired after 65,000 outstanding miles ***