Setting aside whether or not there is a technical advantage to a true mono cartridge over a stereo one the real point here is that if you are going to the trouble of creating a second dedicated mono setup then this is a real chance to choose the flavour of cartridge that best fits with mono recordings. Maybe you prefer the sound of 50s mono played back through a cutting edge MC -- if so great. But for me listening to the Chordettes through my Zero despite it's ridiculous tracking force (3.5g) and far from svelte body has a magic, a fullness, tone and amazing sense of depth that somehow is missing on my far more expensive stereo rig.
And yes the same recording owned in both mono and stereo mixes is often preferable in the mono. Generally 50s and early 60s cutting setups delivered better frequency range (more HF) in mono than stereo (per my conversations on this topic with Tim de Paravicini) so that may be part of the reason. Anyway there's so much to offer and find in mono -- a mono switch is absolutely a good place to start (and helpful for hum cancelling even if you get a mono cart -- that's another topic) but don't give up on the dedicated mono search too soon.
And yes the same recording owned in both mono and stereo mixes is often preferable in the mono. Generally 50s and early 60s cutting setups delivered better frequency range (more HF) in mono than stereo (per my conversations on this topic with Tim de Paravicini) so that may be part of the reason. Anyway there's so much to offer and find in mono -- a mono switch is absolutely a good place to start (and helpful for hum cancelling even if you get a mono cart -- that's another topic) but don't give up on the dedicated mono search too soon.