Motorgliders are becoming increasingly popular, especially in Europe.
The 15M class doesn't support an engine as efficiently. This is why the
18M is being favored. As for performance differences between the pure
sailplanes, they only become apparent when the lift strength falls
below 3 knots. The 18M ship don't run away, but they do start gaining
both in run and climb.

I like the mix of nimbleness and performance of the 15M class. And I
love the high speed performance. If these gliders lose favor, I'll be
disappointed, at least until I buy my first 18M glider and discover its
charms.