I had a 550xp mk1, I let it go to upgrade to the mk2. The 357xp is stronger for sure, but the rev boost feature of the mk1 was cool for limbing.
If you’re looking at 12” hardwood, the 357 has 6+ more cc of displacement and you will notice it. But the 550xp is smoother and sips gas and goes longer on a tank. After using one for a whole day (if/when it works, hot starts aside) the 550xp has its own pluses over the 357xp. But for git-r-done-fast, the 357xp is stronger in the cut when the hardwood gets on the larger size.