Interesting responses, everyone - thank you!
I hadn't considered too much the other 5" devices, but I am looking at them now - there are a lot more options in that size than I realized! Wow! I'm a little leery that they're not something I can go to a nearby store and hold in my hands before buying, but hey, that's what reviews are for.
As for a cell phone, I'm not really inclined to go that route - I've got a very old-school phone (texts, calls, and doesn't do anything else!) that I plan to keep 'til it dies. I'm avoiding a smartphone because I get easily distracted with things already; having the internet in my pocket is a baaaaad idea.
I read maybe 20 pages on the screen when I tried a 300 out at Best Buy - size didn't bother me too much, but maybe it would be different for long-term reading. Hmm. Then again, I think it'd probably be a sacrifice I was willing to make for additional portability - space for items to carry with me is definitely at a premium.
It looks like I should probably do some more research. Thanks for the help!