I recommend an iPad.
I'm not doing it just to give Apple publicity or whatever, but I wouldn't say this if I didn't think I have good reason to.
iOS esp iPad has the widest range of digital painting and graphics-related apps compared to Android. Also, all the three lines of pressure-sensitive styli have the most compatible apps in iOS. This is not necessary, though, but can be very useful.
I also think that iOS devices have the lowest amount of latency.
Another good option you might want to look into are those Windows laptop-tablet hybrids, but I don't know much about those and plus, you may need to carry around a pen tablet for input with you, so it adds to the bulk.
Let me know what you think.