I suggest you to choose sikkim as I have been already there and it is very beautiful and organic. Also sikkim won't cost you pockets too much.
You can get travel operaters easily around Gangtok MG Marg area.. they have well defined packages
Sikkim is good. Its a such a lovely place. One should visit in their life time. Ganktok us so wonderful to spend time. So important places are yungthang, M. G market, changu lake, Nathula pass, New jalpaiguri, backdogra etc...
It's so awesome to visit. Have momos overe there its really awesome.