Hi, As both places are good. But Kasol will be good, You can do Kheerganga Trek also. The Manikaran Gurudwara is best place to go nearby kasol, You can have hot water bath and langer in Gurudwara. Kasol have more hotel open then Chopta. so there will not any issue for stay also. you can visit Prashar Lake also its come b/w Mandi and bhuntar. its also a good place to see. but in january end there may be a risk of snow.
Hope you got answer.
Regards,
Sandeep Parashar
Of the two places Kasol should be good as there are lot of options you can spend time doing
Manikaran
Kheerganga trek
Tosh
Malana
Kasol itself For chopta, your itinerary gets restricted to chopta stay, tungnath and chandrashila treks and that's it. If you want to relax purely and enjoy mountain bliss, chopta is better than Kasol as it gives you more mountain feel (Kasol is much more commercialized) Purely depends on what you wanna do