Hello Aaysha, both the destinations have something different to offer . Kasol offers you a hill station with a lot of serenity and calmness amongst the wild outdoors and during this time of the yea Kasol also might have snow falls if you're lucky . Udaipur on the other-hand is a beautiful city with rich culture and multiple heritage sites and amazing people . I suggest taking your time and deciding if you want a getaway (Kasol) or a sightseeing trip(Udaipur) .
Hey Comparing Udaipur and Kasol is like comparing Burgers and Pizzas, we love both and can't choose one.
Udaipur is filled with gardens, palaces and historical stories which are worth erupting curiosity while Kasol is filled with peace where you can spend your day at moon dance cafe eating brownies and can do to various other cafes. If you are into smokeups Kasol has no competition, cafe with trippy interior, Bob Marley songs are the best to spend the whole day and you can also spend your time trekking some small trails which you can discover by yourself or can choose a trek group to be with!! In either case I'd recommend you to stay in tents, Udaipur has this wonderful place where they offer tents instead of room, google it up and you will be amazed by the ambiance, it's the best and so is Kasol!!
Both these trips offer different experience. Udaipur is a colourful city, frequented by tourists as well as backpackers. You can enjoy forts and palaces here and indulge in an ancient culture. There are many food and shopping options too. Find more here: https://www.tripoto.com/travel-guide/udaipur Kasol is meant for a leisurely backpacking trip. You can enjoy trekking in the Himalayas, and a quiet, peaceful time chilling on meadows, if it's not snowing. Find more here: https://www.tripoto.com/travel-guide/kasol
Both places are totally different from each other. If you want a peaceful, refreshing getaway in the mountains then Kasol would be ideal. However, if you want to have more of a sightseeing, Udaipur would be the right option. While you can visit the city palace, saheliyon ki bari or ferry ride to Fateh Sagar lake(beautiful man made garden amidst an artificial lake) in Udaipur, you can trek/hike and enjoy a panoramic view of the Himalayas in Kasol.