Actually a very tuff question because both have their own different charm.
I would suggest you to make a plan of places on both sides and then decide which one attracts you.
Hi Pranay, I would say choose Sikkim because of its beautiful hill stations like Gangtok, Ziro or Darjeeling. But you can also visit the beautiful and untouched hill station in Assam:
Haflong – The most splendid and serene one. Diphu – A peaceful retreat for nature lovers. Umrangso – An unspoilt picturesque beauty. Maibang – Famed for relics of Kachari Kingdom. Happy Traveling!