Well I’ve ran them both extensively. Pros and cons of each in my experience.
ECHO CS590
pros
Reliable
Easy to work on/modify
Easy to start
Easy parts availability
Loads of torque (like big block amounts)
Best bang for the buck on a new saw out there
AV is great
wide clutch cover
There is more but those are the big ones
cons
70 DL chains unless you slap on a common bar
Spur sprocket
A little heavy
Plastic clutch cover
HUSQVARNA 555
Pros
Light weight
Auto tune
Rim sprocket
Standard bar/chain
Reliable
Even better AV
cons
Starting process ie primer bulb
Takes forever to break in
Outboard clutch
So they are both choked up from the factory the Echo more so. At the bare minimum delete the tabs on the carb adjustment caps so you can tune it correctly. With a muffler mod and base gasket delete it will flat out run circles around a lot of saws.
The outboard clutch on the 555 drove me nuts. Its for better felling balance. It wants to tip over on the clutch side for felling and limbing. It is more maneuverable but every time I set it down to move something I would reach to pick it up and it’s laying on the clutch side. It made chain changes a ***** and it clogged like a gas station toilet when noodling. It has a very restrictive muffler too like the Echo. Both benefit greatly from just a muffler mod.
So in summery in my opinion the Echo is a better firewood/bucking saw and the Husqvarna is a better felling/limbing saw.