There has been lively discussion on the board in the past so a search may help you. Here is my take.
o they are indeed built on the same hull
o 300
the original model (1989)
walkaround
larger cockpit
smaller helmdeck w/ less seating
starboard helm
ez helmdeck access
more cabin space
cabin and head side windows
better battery access
overhead boxes
less expensive (~20K)
more fishing friendly
o 305
around 3-4 years old
express
smaller cockpit
larger helm, more seating
center helm
tight helmdeck access (due to center chair)
popup box
new options (chill plates, mp3 support, etc)
more family/cruise friendly
There are owners of both here. I am a Marlin person since it suits my needs much better. Southern Hunter is one of the happy 305 owners.
We can answer any specifics about either model. They are both terrific boats so choose the one that suits your needs. Now is a VERY good time to purchase IF you don't need to also sell a boat.