The two destinations are very different from each other.

In Nepal, you might want to spend your whole week in the Kathmandu Valley. There is plenty to do and see there. Here is my trip report from a few years ago that covers all of the heritage sites in the Kathmandu Valley.
www.fodors.com/community/asia/kathie-returns-to-katmandu-2008.cfm

We were in Sikkim two years ago.You are right in the Himalayas, so the views are spectacular. Sikkim has many Tibetan Buddhist monasteries which are wonderful to visit. I's recommend two areas to visit: the Gangtok area and the Pelling area. While we went to Darjeeling as well, we weren't impressed with it after being in Sikkim. We stayed in Heritage hotels, both run by the Elgin group and thought they were lovely.
www.fodors.com/community/asia/sikkim-darjeeling-and-kolkata-an-unusual-first-trip-to-india.cfm

Photos of all of our trips are at www.marlandc.com

hi

Yes two destinations are really different ....If you would come to Nepal then it would be really nice and wonderful to visit " Chimkeswori village trail " which is really suitable to explore nature people and religion of the area ....offers a magnificent view of snowy mt. range along with Everest . thanks

regards
Dev