For a short trip (weekend trip or long weekend), Rishikesh is a good place to visit from the plains. You can enjoy activities like rafting, cliff jumping, trekking and temple hopping here. Manali is ideal for a longer vacation (4-5 days or more). You can enjoy activities like paragliding, rafting, trekking and zorbing here or just enjoy a snow fall in the winters.
Rishikesh is good for a short duration trip. Rishikesh is famous for yoga and adventures like rafting and bungee jumping, but rafting is closed in this season whereas manali is a proper hill station , much colder than rishikesh. One can see snowfall in manali .