In my opinion based on using both, 550xp as well as 543xp, i say that they share pretty much nothing, and yet they feel pretty darn nice, both of em. The 543 is clearly more compact and such, the space between fronnt handle and the throttle is shorter and in some ways...more aggressive, i think 543 has better ergo´s for fast removal of branches.
With 13" bar and filled tanks, its back weighted, but with 16" or so, its much more balanced, when the chain & bar are .325" / 1.3mm....with 15-16" bar length and 1.5mm chain its at its best i think. But in my case, 13" bar is enough for the tasks that i work around.