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