The 2003 proposal, alternative C-Prime, was to be mostly in an open trench to Blue Grass Road, then elevated to Southampton Road. I think that the current proposal is agnostic as far as the kind of grade separation. The
proposals made by the Department of City Transit from 1912 to 1923 were for an entirely elevated line, which I believe met with some protest. I believe that later proposals were for mixed subway and elevated lines.
I prefer a heavy rail line of some kind to provide long-term potential capacity and to use the considerable surplus capacity available on the mainline, particularly the express tracks. The half-measures of a bus rapid transit or light rail transit line would require a transfer and presumably longer trip times as well as some susceptibility to traffic at intersections, which would impair its utility, thus suppress ridership.