I haven't been able to tell a difference using the Scangauge, but I have to find an open stretch of road for a real test.
Recirc cools down the inside air using cooler inside air instead of the outside, thereby making the system more efficient. The AC compressor will engage and disengage more often when it's hotter inside, so I would guess that it's more efficient to run with recirc on. The difference shouldn't be very great, maybe 2-3mpg. With that little difference, what matters is that you're comfortable inside. Just remember that recirc will recycle CO2 and not let in new air, so you don't want to run recirc for 4 solid hours... fresh air is our friend.