Now volvo or equivalent buses are available for all major tourist destinations across India. Some are purely sleeper class, some are purely semi sleeper while there are also buses which contains both types of seats together. Now question is how to find the best seat. If you are traveling solo, then I would recommend single sleeper box, which has maximum area per person as compared to two person box. It comes with some cons as well like it is mostly located just above the front tyres so you will feel each and every bump of the road.  I travelled to Vaishno Devi in sleeper class and it is very difficult to stay still in sleeper seat when compared to seating position in hilly portion, where you will be keep falling to one or other side on each turn, there no grab handles to use while seating on your berth.  So my conclusion is to have sleeper berth only on plain areas not on hill station journey.