ive got a 308 with supra 5 speed and 3.55 diff. 5th gear is heaven. If you want to stick with manual and tow whilst still having the ability to hit the highway, i think the 5-speed is the way to go. Tall diffs are shocking for towing. My bro has blown 2 aussie 4speeds in two weeks towing a small boat with a 2.78 diff. Never had a problem until he changed diff ratios. he went 3.55 to 2.78. Even without a boat on the back he had trouble taking off in 1st with the 2.78!

Id agree with the others above with regards to ratio, 3.08 would probably be the best all rounder. maybe a 3.36 with a supra 5 speed in your case

let us know what transpires