Forerunner 305 is a great improvement over the 201 for trail runners and a slight improvement over the Forerunner 205.  First, the GPS receiver maintains coverage much better in canyons and heavily forested than the Forerunner 201 or 205 did. (It is my belief that this has been greatly improved due to firmware updates.)  Second the controls have been better positioned on the watch to aid in use while running on the trails.  The last great improvement with this watch is that it seems to synch up much more quickly with the satellites than the Forerunner 201. Additional improvements are that it can now be charged and connected to the computer using a USB connection (versus the serial port connection of the Forerunner 201).  The Forerunner 305 also comes with much better training software and the ability to create "courses" on the watch.  I have only created a limited number of courses on the trails and find that I primarily like to use them when I am trying to meet or beat my performance on a particular trail or run.  The biggest difference between the Forerunner 305 and the Forerunner 205 is the HR rate monitor.  I did not buy the watch for the HR functionality but I have definitely learned the great value to having that information available both during the run and post-run in the software.

My biggest recommendation to all users is to frequently check for updates via the computer software for the watch's firmware.  I have noticed that Garmin is very good about releasing firmware updates that help the watch's performance.

Pros: Heart Rate Monitor, Faster synch (over the 205), better coverage in canyons and densely forested areas, more usable controls, better handling of highly twisty trails.
Cons: None.