I don't think the comparison with lenses is right, as there are outstandingly good (and bad) lenses both in fixed focal and zoom forms. Lenses may also have very different characters (color rendition, bokeh, micro-contrast, aberrations, sharpness at various apertures, vignetting, color cast, etc) so the choice is not always between better quality and flexibility.
Monochrome sensors used to have a sharpness advantage (no low pass filter), better contrast and low light performance for the lack of a color filter array (sometimes even lack IR filters). Today the differences are probably minor, as sensors get better and the software is more efficient at interpolating color data.
You may find meaningful differences for scientific applications, but for photography they are pretty much the same thing. Though if you only shoot in black and white, the monochrome sensor saves you the conversion step of the RAW file.