I have used Fujitsu ST4xxx series Tablets, a Palm T|X, and a Sony Reader.
At the time I found that Fujitsu tablet most excellent for ebook reading using eReader Pro and MS Reader. And the programs are optimized for Tablet PC IIRC - at least both work nicely for either portrait or landscape mode. I usually used it in Portrait mode.
So why did I go on to get a Sony Reader, and use it exclusively for pleasure reading now? Two reasons:
a) I'm married, and the Tablet screen light, even on dim, disturbed my wife's sleep. No matter how I oriented myself in bed, the light flowed from my screen towards me and thus to my wife. A backlight for the Reader doesn't do that, and I can use a dimmer bulb than the lowest brightness setting on the Tablet. (Plus the frustration of having to play with the Tablet's brightness settings twice a day - not a long process, but tiresome in repetition.) Now she's happy at night, and I am too.
b) While the Tablet is holdable in one hand, it's still pretty large and cumbersome. I didn't mind it - and preferred the larger screen to any handheld PDA. But when an alternative became available, I snapped at it as quickly as I could.
and a bonus c) The hard drive burned out on my first Tablet after about eighteen months of hard usage. I don't have to worry about a hard drive with the Reader.
That said, a Tablet is still a very good choice. You have color if you wish and if supported, and display of PDF in letter size is very readable and nice. Even though I've switched to the Reader for all casual reading, that's a very individual decision and mileage varies much when considering the decision of what to purchase.
Best, of course, is if you can find what you're interested in and try it out before buying. Then you know you've gotten what you want.