Depends upon your choice, if you want calm peace and hippie type feel with mountains, river and lots of waterfall + small trekking in nature then Rishikesh is calling you. But you want some folk music and traditional Indian feeling with lakes and forts, then Udaipur
(PS: Rishikesh is still unexplored and its really beautiful, stay at tapovan and explore that area. Phool chatti waterfall and neer waterfall is must )
Both are very different from each other. if you are an adrenaline junkie then go for para gliding, white water river rafting, bunjee jumping in rishikesh. you will love it there. the weather is pleasent in most time did the years.
Reach Dehradun by taking a flight or train then take a cab or local bus to Rishikesh. but if you want to experience something straight out of a history book, visit Udaipur. you will get to see both medium level mountains and lakes. oh! breath taking lakes, my friend.
The City palace is so Regal. You can visit bahubali hills, Sajjangarh palace, Fatehsagar lake, Pichola lake. so many places. A 3-4 day trip is Sufficient to cover Udaipur.