As far as effeciency a large single is better. More Hp. more tork and higher rpm.
But these twins are real gems.
They run so smooth and transition very well.
Both pistons counter balance the other.

Although the GT-80 is used as a race engine by rcignitions the ones I play with are great sport engines.
I recently pulled a G-62 off a plane because at idle it shook the plane more than I liked.
The motor mount that I knew was tight came loose in flight.
I haven't flown the plane with the 62cc Echo twin yet, but I have told people to come over and watch this thing idle.