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.
Anup sir is right, finding a room in either of the hotels if you haven't prebooked is tough. Personally I found Manalsu's garden and the hotel building quite beautiful and would have loved to stayed there but sadly I hadn't prebooked and there wasn't any accommodation available in early June.
There are so many hotels in Manali these days that even on peak tourist season you will get a room, though it might not be exactly what you want and would cost a little more.
By and large, if you land up at Manali by bus, do not settle for a hotel with the touts. Instead, go yourself and you will get rooms at the right cost, rather than with the bluddy tout's 40% added on!
However, I know this is easier said than done. What is required is for one person to stay with baggage at the bus stand, and others go in search of rooms.
If you do settle with a tout, check in for ONE DAY only. Next morning, re-negotiate the rate at the desk or simply move elsewhere.