Here is a similar thread I started recently.
http://forum.getdpi.com/forum/showthread.php?t=11679
The 3 main differences between the 3541XLS and 5541LS are weight, max. load and height.
The 3541XLS weighs about 2kg, can carry 18kg, and is almost 2m tall (78").
The 5541LS weighs about 2,8kg, can carry 28kg and is only 1,50m tall (54").
I personally find the 5541LS too short. As soon as you are standing on a slope, you will have trouble getting significantly off the ground. I also don't think that you need a 28kg max. load for MF. Maybe with a 1000mm lens or 8x10 camera.
My other consideration was the 5561SGT, which is giant, about 2,6m, but you need a step ladder to use that at max extension, and is weighs about 50% more than the 3541XLS, so I decide that it was too specialized for my uses.
I ended up with the 3541XLS which arrived just this past week. I didn't find the time yet to go out, but perhaps this weekend. It is very robust and the operation is such an improvement over my old Manfrotto 055C. I must admit to being surprised at the diameter of the top plate. The tripod diverges significantly towards the top. I think 2/3 the diameter would have been sufficient. My rather large Manfrotto 405 has loads of room up there. Anyway, it is what it is, and I am happy with my choice so far.