3.55 vs 3.91

If you have the A999 with the low gear set (first and second) then the 3.55 will feel like a 3.91 in a 'regular' automatic. I think it is about a 20% difference but I don't recall exactly. If you will be spending any significant amount of time over 50 mph, go with the 3.55.
I have 3.91 gears in my 65 Plymouth and have gone on a few 300+ mile trips. (I have had these gears since 1985) It can be a little monotonous but something you just live with.