With new ownership, Maya Island has been investing in new equipment and new facilities. They've got some new planes, a nice new "terminal" in Placencia and a big, newish building at the municipal airstrip. They've also been expanding their service -- to San Ignacio for example.
Once I'm in the plane, I can't tell much difference in either one. Schedule and price -- those seems to be the main things these days, on airlines anywhere including Belize.