I have Screw 145' Eco 4X4 with 3.55 and it tow just fine. I recently towed 4,500 lbs from SA to Houston a few months back at it didn't even budge from 75mph going up the hills. It only downshifted into 5th grear once on a grade 3 hill. The 3.55 and 3.73 have the same max towing capability at 9700 lb 4X2 & 9600 lb 4X4 unless you get the max tow package which is 11300 lb. That info is found here http://www.ford.com/trucks/f150/specifications/towing/
. The 3.55 will net a better mpg then then 3.73 if driven identically unloaded but not much. I have driven both and cannot tell a difference at all in acceleration and towing. With this much low end torque in the Eco maited with the 6R80 transmission, the two rear gear ratios will be hard to tell apart unlike other engines where it was more noticable. With the amount of weight you will be carrying, then you probably could not even feel the difference. It pretty much comes down to personal preferance. If you need more info on how well the 3.55 tows then read this article http://news.pickuptrucks.com/2011/04...-6-part-1.html
. Both Ecos are 3.55 with one pulling 9,000 lbs up and over the mountains. Hope it helps. Don't feel bad, it took me weeks to decide which gear to buy.
Edit: Also, if you read the pickuptrucks.com review you will notice that 3.55 was a bit faster in 0-60. The 3.73 was 6.82 with the best of 3 runs and the 3.55 was 6.79 with the best out of 3 runs per this review.