The two leading brands in Chainsaws both produce exceptionally good chainsaws. It is one of those happy instances where making either choice is correct.
There is one significant marketing difference, though: Stihl is only available from registered dealers, while Husqvarna chainsaws are available online.
Husqvarna claims that their X-TORQ feature ensures that their saws have better fuel efficiency and, consequently, lower emissions. Husqvarna saws also have the reputation of faster cutting speeds through the wood.
Stihl chainsaws, on the other hand, tend to be a little lighter with smaller fuel tanks. They also tend to have a more competitive pricing structure.
It ends up as a matter of personal choice, but both companies offer great quality products. The more difficult decision is to get the right model for your specific requirements.