I haven't had a chance to mess with the 300 yet, but from everything I've read about, I have little interest. It seems too cluttered with effects I don't need and lacks the one I do -- reverse.
My 50 has been lacking in sound when I turn it on for the first time since it's last been off and needs to be turned off and on for it to work. I just reformatted it and it still had that issue...I'm going to spend some more time trying to figure out that issue. Plus, it's beginning to get noisy on it's own (eg. I'll turn the instrument volume down all the way and there'll be needless noise that disappears when I turn the 50 master volume down or take it out of my chain. Sad to see this happen, I fucking looove this thing and am dependent on it my band, 3dCosby.