Lot's of good comments here but I'll chime in since I seem to be hard on tripods.
The weak area in the design of the tripods is the metal casting at the top of the leg sections. CF is quite strong and needs a lot of force to cause damage. The metal casting at the top of the leg section is thinner than the CF wall, and can be fractured with enough force.
The bottom leg section does flex a little under a heavy weight. I had a 600mm f/4 on my 3540LS. The rig was on a frost covered wooden deck and there was visible flexing in the lowest leg section with very little pressure. On a firm surface this is not really an issue.
The XLS model could be a good choice but involves a lot of compromise. The longer length is great for unlevel ground, stream photography, or shooting subjects that are near directly overhead. Nikon pro Bill Lea is about 5' 8" and uses an XLS, but he does a lot of photography on ground that is not level and around streams. I wish my LS model was 6 inches taller even though it is perfectly adequate 90% of the time.
The XLS is much longer than the LS - which for me means it does not fit in my normal luggage. That's a real problem for travel - and the reason for choosing the 4 section tripod, so I would not consider the XLS suitable for most air travel.
The positive on the XLS is that it usually operates like a Series 4 with the bottom legs not extended. That does add stability. Even the LS models do not require the full leg extension all the time, and partially extending the legs adds stability.