Hi--
The topic has been covered before so if you wish for some reading material you can search for it.
Basically the 576 is bulkier and heavier with more power. It will balance a longer bar better. it also has a fatter torque curve, than the regular 372 at least. I have both right now. the 576 is less suited to limbing. I do really like the sights and the lines on it for fellign though. ergonomically it is a great saw.
That said, the 372 is easier to throw around and limb with.
The 576 is available in Autotune, which is a computerized/automatic carburetor that is constantly (tens of times per second) optimizing itself to the current conditions. Most users report a performance gain as a result of autotune, in the cut.
Most, if not all, 372s you find at the dealer will be x-torques. The x-torque is a modified 372 which meets post 2010 emissions requirements through the use of stratified engine technology or "strato" engine. Again a google search is probably your best bet here to google strato engine, redmax has a good page explaining most of it, and there are videos on youtube. This isn't particularly radical technology, it's been in equipment since the late 1990s and is becoming the norm.
Most will say there is little to no performance lost, some performance gain with the x-torque with the 372. The 576 also an "x-torque" saw.
For clarification, I have a 372 (non xt) and a 576 (non autotune).
It will be a tall order to find someone who has run all 4 quite a bit.