I have the ZT300, FlatLander, and I really want the ZT0350CB. The 300 adds 1/2" more length, more width and thickness at the blade. The handle has the titanium side and G-10 side with steel liner, framelock and the Hinderer Lockbar Stabiliser. The G-10 side has a macro-texture on it not like the micro-texture on the 350 and although it is kinda smooth, the larger texture grips well. Also, the 300 has a larger pivot screw that socket/open-ended wrench, not Torx/Hex.
The assisted opening action is not quick at all but it shakes your whole arm when it clicks locked with a solid "CLACK" (not a "click" like other AO's)

. A very cool opening IMHO. You know how the hero in some movies get the slow motion intro with the bad-ass pose at the end, the AO on the 300 is like that

.
So it does adds a some things but functionally speaking, the difference is just size, different steel and framelock. I would definitely recommend the ZT300 though. It is my favorite knife of all the knives I have or have had. :thumbup: