I see a few aspects to this question:
Does an extra seat add comfort?
Obviously extra comfort is subjective. Megabus seats generally have really little legroom. Having two adjacent seats can allow you more leg room and general personal space.
I've travelled on lots of buses in the UK. I've always encountered buses with either no middle arm rests or moveable arm rests that raise up. Whether the arm rests will be flush with the seat back in the raised position or whether they'll still stick out a little bit depends on the bus.
An additional feature on most new buses in Europe is that the aisle seats can be moved a few inches into the isle, away from your neighbour in the window seat. This helps provide more personal space if you just have one seat, but it's not clear to me whether this is common on Megabus' UK buses.
What's Megabus' policy on booking multiple seats for one person?
I made an enquiry to Megabus, the response indicates they don't support customers booking two seats:
Unfortunately its not as the seats are not numbered so passengers choose there own seats when boarding so you could not stop anyone from sitting on the seat next to you.
Please accept my apologies on behalf of megabus.com for the inconvenience caused.
Across the pond, Megabus actually suggests booking multiple tickets for those passengers who need additional luggage allowance.
Will you be comfortable asserting your claim to your extra seat?
I've been in the situation before where I had two tickets (I got the extra one due to baggage rules) and I was looking forward to having extra space. I felt that I was entitled to two seats since I'd paid for them. But the bus was overbooked, so it felt rude not to let someone else use my extra seat, so I did. This type of situation can come up. In general it's worth considering whether, for instance, you'd feel comfortable asking fellow travellers to move so that you can have your extra space, given that if you don't board at the original station of the route there may not be two adjacent seats left.
Personally I'd rather have less space than come across as a jerk, but I'll sure be hoping the bus isn't full and I get two seats.