I haven't used the 300, but I got a 350 to replace an astak that I broke. I like the 350 a lot, but it's not so much better that I would have replaced my astak if it was still working. I'd imagine that it's the same with the 300 and 350, if you're happy with the 300, I don't think there's enough difference to change. If you're buying new, I'd go with the 350.
My US$0.02