The PB360 is the size of CD jewel case; try holding one in the hand to see how it feels.
The PB301 is closer in size to a DVD case.
Reading on the PB360 is like reading a (very thin, very light) paperback. Since you have fine-grained font size control for DRM-free ebooks it really is going to be up to you to choose higher pixel density a font face/size combo that suits you.
6" in screen devices are not cumbersome but for *my* tastes, 5" is the sweet spot: I have two 6" devices, plus a 4" PocketPC, but these days I do the bulk of my reading on the PB360; it weighs about the same as the PocketPC with much better battery life and it has slightly sharper text than the 6" screens (smaller size + same resolution = higher pixel density).
If you still can't decide for the 360, you might want to wait for the 302 which will have better ergonomics than the 301 in a six inch size. But it *will* be more expensive because off the WiFi and touch screen.