The 18's have a few more tire choices from what I have found (have considered switching to the 18's from the 20's), and it seems most have a higher payload rating as mentioned (20's are NOT available on the 3500HD unless something changed recently).
Don't rule out a 3500HD either! When pricing our '13 (14's also) the 3500HD was actually a little cheaper when optioned the exact same! Just couldn't find a 3500HD like we wanted. Couldn't hurt for the extra payload that would be available, especially if you're going to be doing a lot of traveling.