Here's a speed graph that I came up with. X axis is RPM bundle (1000-3000 in 200 RPM steps), Y axis is speed in MPH.

This is based on the 42RE- 2.74, 1.54, 1.00, and .69, with my tire (265/70-16). The bottom dot cluster is 1st gear of each RPM pack, the second cluster is 2nd gear, ect.

Right now, 60 MPH is ~1600 RPM. 3.92 would make this ~1750, and 4.11 would make it ~1900.
70 MPH is ~1900, 3.92 would make it 2050, 4.11 would make it 2200.

It's pretty safe to say, that 4.11's are worth about 10mph at highway speed. I'm thinking that I might as well get the 4.11's.

 

I dunno about the 3.9 V6, but the Hemi has a "sweet spot" at right around 2200 rpms and it'll actually get better fuel economy cruising at that rpm than it will at say 1800 rpm. Not to mention being more responsive...