I have several friends with a PRS-350 and as an owner of a 300, it was interesting to compare. The 350 has a better screen, crisper, brighter and it is touch. Over 1Gb more storage as well. As far as I was concerned, my 300 is still just as nice to use, regardless of the extra features of the 350.
I would no worry too much about upgrading if you need to, so enjoy your 300