If you require portability with high end specs and a disc drive: laptop.
If you require portability, need it for work purposes but don't need disc drive/high specs [ie you are not a designer]: netbook.
If you don't need it for work, but for browsing on the go and such: smartphone.
Personally the only viable use I see for tablets is for use at home, basically as a big smartphone. I would keep one in my living room to use for light browsing instead of traveling upstairs to a full desktop every time I need the internet. If I can't carry it in my pocket then it doesn't have 'casual portability'... sure it's possible to carry a tablet in a bag, but the same can be said for a laptop, whereas a smartphone does the same stuff as a tablet and is smaller.
For gaming... it depends entirely on your tastes. I am not fussy about having epic or deep experiences for gaming on the move. If I had a 3DS or Vita it would be used in places like on the train, but in those situations I would be perfectly happy with little flash game-like experiences.
The reason to buy a 3DS or Vita is if you want to play the games that are on them... simple as that.
