Hi... the distance from dehradun to mussoorie is around 1 hour to 1 . 30 minutes at least try to stay a night in mussoorie would be great. it's on a Hilltop so you will get better view. rest dehradun is also beautiful city. so many places to see in dehradun itself. but atleast you should spend a day in mussoorie as well. visit mall road, gun hill point, kempty fall, bhatta fall etc in mussoori then come back to dehradun next day. visit buddha monestry, sahastradhara, lachhiwala, tapkeshwar temple etc. regular buses & taxi, cabs available from dehradun to mussoorie. but for better experience try to stay a day & night in mussoorie
Mussoorie is 2 hours away from Dehradun. Since your flight is from Dehradun, I would advise you to go to Mussoorie stay, see places and come to Dehradun see places and then take a a return flight. Both are lovely places to visit, that you shouldn’t miss exploring. Another option is based on your number of days of travel in both the places. If you are planning two days in Mussoorie then you can plan a day stay in Mussoorie explore and then come to Dehradun. Then you might need a day to explore Mussoorie. Suppose you plan 2days in Dehradun then you can plan a day stay in Dehradun and cover Mussoorie too.