Piston porting is the most common. Some saws such as the small Echo have reed valve induction which generally gives stronger torque but is more expensive. Emissions also seem to be an issue with this setup though, most manufactures have dropped it.

The difference between the two designs (PP vs Reed Valve) is how the engines are introduced fresh air and fuel to run. The piston ported engines use the skirt of the piston to close off the intake port. Reed valve engines use a one way reed valve to accomplish the same thing. Reed valve engines usually have the carb feeding the crankcase whereas PP designs have their carb up on the cylinder. Without going into a big discertation here about engine design, both the Piston Ported and Reed Valve designs are proven. Reed valve intakes are used on all the old Homelite, Poulan and Mcculloch chainsaw engines. The brand new Shindaiwa C4 engine which is a hybrid four stroke design uses two reed valves. Here is a link where you can watch the engine work in animation>http://www.shindaiwa.com/nam/en/technology/c4/index.php. After the page loads, left click on "View C4 Technology in action". Hope this helps.
Best wishes,
Bob.

Even the reed valve induction 2 strokes use piston porting to control transfer from case to cylinder and for exhaust. Like Canguy said it boost the torque of low cost relatively low revving smaller saws so on a few that is about the only place you see them. The reed valve can't function much more then 8 or 9,000 revs.

Total piston port engines rev faster and is more in line with modern high revving saws which run 12 to 14,000 revs.