With the right interface, I think the synth can be more expressive. Look at the Haken Continuum or ExpressiveE Osmose - both can be used with something like the Expert Sleepers FH-2 to get MPE data to the modular.
I do see your point, and agree the amount of articulation you can do with guitar is hard to beat, but I do think a synth can win, if the setup is built for it.