I can't comment on the 11's, but I simply love my 13.. :) I've only towed with it one time so far and it's only got 3500 miles on it, but it's a puller for sure!
Mine has the "Max Tow" option which gets you a couple hundred more pounds of GVWR and payload (I've got #1900 of payload per the sticker.. :) ) and those nice big a$$ tow mirrors, which are great by the way!
I was shopping for a used truck initially too... But when I saw my brand new 13 for $33k and a used 11 and a 12 for around $30k I decided to go with the new one! Those used ones had over 30k miles on them already too!
Good luck!
Mitch