Arun, as a general rule, Tourism Department hotels are always good - affordable, nice location, parking, usually a garden too, good catering and some 'minimum' standards are invariably maintained.
Of the two you have mentioned, Beas has some rooms with a view of the river while from Rohtang Manalsu none of the rooms have a river view. The Manalsu maintains a lovely garden and is a wee bit more upmarket.
During season you are unlikely to find any rooms without having booked in advance.
Across the river, there are many hotels (all budgets) and I would expect that after some legwork, you'd be able to find something.
The conjested alleys across from the bus stand have many 'budget' hotels.
There are hotels scattered all around Manali. You ought to reach around noon to be able to look around and find something suitable. If you reach late evening, your choices are severely limited.
