if you got a real good deal on the 550, i'd say go for it. it is a good saw, just a little heavier than the others in it's class.
if you're used to running an older chainsaw, you might even like the feel as it would be closer to some of the older 50ish cc saws, just quicker and smoother. i know the av is very good. it's a smooth saw to run. you have to remember, this saw is being marketed in the same class as the stihl ms290. this saw is so much better than the 290. hands down.
i would put this saw in the same catagory as the dolmar 111. very good firewood saw that weighs a little more but is a smooth runner.
dolmar has used the excuse in the past that the small amount of weight they keep in the saw helps reduce the high frequency vibes through absorbtion.
that being said, the cs500 is just lighter, faster and just as smooth. it would be the one i would take into the woods with me if i need to run a saw for 8-9 hours a day everyday. the 550 would be a great choice if you were running it for 5-6 hours a day every few days for the usual firewood cutting.