Both are good choices but you should go to place which is accessible to you easily and faster.
Options in Himachal Pradesh is more though.In north east gagtok seems the only best option if you want see Himalayas.
If Himachal Pradesh is nearer to you, then further your travel route by making your way to North East. Its the season of Festivals here. From NH7 weekender to Ziro music festival to Mechuka Adventure Festival, you will encounter all the colorful vibes and meet many interesting people, even make life long friendships on your way. The beauty of North East will surely take your breath away.. may take some time to get here, but the results will be amazing. Safe Travel!
India !! where you can taste so many experience in every State of this country..If you want to visit Mountains, then you can visit Himachal or North East or Uttarakhand or Kashmir..as well as you can try adventure sports over there..
All those things are depends upon your budgets. If you want to chose between Himachal and North East, then I will go for Arunachal Pradesh..
You should visit Himachal if you have less no of days for vacations. and Should explore north east if you have more than 10 days for Northeast. In that way you can cover multiple states of North east. While in less no of days you can go to any place of Himachal pradesh.