Info I've gleaned from somewhere is that the plectrum banjo started with some players in the late 19thC taking the 5th string off and playing with plectrum rather than fingers. When this became popular, makers started making banjos like that, and calling them plectrum banjos. Any of the 5-string tunings will work (minus the 5th string, obviously), which gives you plenty of choice - you wouldn't believe how many there are. Plectrum guitar is the same, but with a guitar body instead of a banjo pot. Not much point in playing it fingerstyle, though; you might just as well stick with the 5-string as do that.
John Kelly.