If you want to draw on it, you should go for wacom penabled tablet. Not all the device have it. Some have the N-trig system, that allow you to draw too, but it have less pressure sensitivity.
There are some android tablet and windows tablet pc, we can include transformer too.
Android tablet are cheaper but often don't offer a keyboard with the screen. There are some good soft on them for sketch. But you can't work a final big image on them. They have the greater autonomy battery life. There size are 10-11".
Tablet pc are good, you can find some refound items for a good price. A lot of brand created a tablet pc, so you have a big choice. They have less autonomy but you can use all your normal software on them, they are more powerfull too. You can find some at 11" to 12", maybe more I don't know all of them.
Every of them are easy to travel with, I can try to give you a (long) list but honestly it would help if you explain a little what will be the purpose of the device

It is for drawing from life outside ? Inking ? Sketching ? Painting ? Multi-use ? Do you want a keyboard to work as a latop or is a tablet enough ?
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