I prefer ePub quite a bit myself. Fully justified text on the 300 means you sometimes get sentences with 3 words, and big gaps in-between them. (Even at the smallest font size.) It's very jarring to read fully justified text on the 5" screen.
I may feel differently when using a larger screen.
Also, if you upgrade to a new reader in the future, even if it's Sony, it may not support LRF, and you may have to convert them all anyways.