Any more details that you know about thats not evident from the 2 liner explanation on the gitzo webpage? For starters - it is 0.7 inch taller when folded up, and weighs 0.1 lb more than 1540T. However, it does have a huge difference in max load capacity 17.6lb vs 9.9 lb.
(I do remember reading an another post that the 1540T has the same 17.6lb load capacity as well).
In basic comparison of all 3 traveler tripods, it appears that 1550T is the smallest sized (folded length) with lowest load limit, while 1541 T is the tallest folded up with best load limit.
Go to the web demonstration of their titanium traveler model. I've had a long talk with Gitzo Uk about it. It's just about identical to the titanium - but not titanium and no ball head included and no leather case