It completely depends on the places you are visiting. in Himachal Pradesh most of the places are scenic and beautiful while in Uttarakhand some places like Byasi, Chamoli, Auli, Kedarnath, Valley of Flowers are equally beautiful and peaceful places.
So choice is yours.
Himachal is best for all time because u feel the nature and peace in the Mountains U also have ti experience the new things if u have a lots of time so than u can travel to places like chamba , kinnaur and stay with the local people without the internet and mobile
Both are hill stations..in summer you can go anywhere between this two states.. You can try Kasol-Tosh-Kheerganga-Pulga or Mcleodganj-Dharamsala-Dalhousi at Himachal
You can try Rishikesh-Haridwar-Mussouri-Derahdoon-Auli at UK. If you go Uttarakhand then try Bunjee at Rishikesh