E-reader (Kindle keyboard) because I prefer the non-backlit display. I understand the new PaperWhite Kindles are lit without really being backlit and are pretty awesome, but I'll keep using the one I have until it dies. It is perfect and I don't need anything else on it. I have a separate 10" tablet for other stuff, but I don't enjoy reading on it. Among other things, it is significantly heavier and harder to hold in one hand, while the Kindle is perfect for that, even with the case.
If someone could only afford one, however, I'd probably suggest a tablet. But I'll never go without a Kindle again, and I held out quite awhile thinking I loved physical books too much.