If you want to use pdfs or comics where you want to zoom in and pan, then go for a 600. If you want text only, then the 300 seems fine for me. But with the 300 not having a touchscreen where you can move a zoomed pdf to the place you need is similar to my old 505. Sometimes you have columns of text that cannot be read as you have to switch between two halfes of a column with the section separation right in the middle of the column.
All tradeoffs and benefits of the 600 to a non touchscreen apply also.
