Both places are Nice. But In mid may u ll get haevy rush there. if u want to enjoy adventure sports then just visit rishikesh for rafting , bunji jumping, zip line and camping . and if u want to enjoy hills with activities then go for manali. u can enjoy rafting , biking , paragliding there.
Rishikesh , if you are looking for adventure sports than Rishikesh, and if you want to enjoy your morning tea in a cold and cosy place than manali would do the thing for you