Buy Mono vs. Stereo LPs for Stereo Rig

For best results Mono pressings should be played with a Mono Cartridge.

Most mono cartridges are equipped with a dual set of output posts, which means that both inputs on a stereo preamplifier will receive the exact same signal, this will result in a "soundscape" thats portrayed exactly midway between the two speakers set up for stereo playback.

In my opinion there would be no need to alter a Stereo setup in any other way than to have for an example a tonearm with a detachable head shell, or removable arm wand like the Graham mentioned above, to facilitate an easy cartridge change between Mono and Stereo

Good Listening

Peter

Be sure NOT to play stereo LP's with a MONO cartridge. It will damage the LP. Playing mono LP's with a stereo cartridge is not a problem, damage wise.

As to the OP question, there is no definite answer. The original mono recording in theory should be the best, BUT sometimes the reprocessed (especially on a reissue) for stereo may sound better. I don't know of any sure way to know which would be best, although one would hope that the original, if recorded in mono would sound the best, especially with a mono cartridge.

Good listening

The mono reissues are cut with a stereo cutter, so can play those with a stereo or a modern mono cartridge. Modern mono cartridge means, the stylus is identical to their stereo brother but you will have a much better performance. Less noise and that is the real advantage, the tones have colors which are hidden when using a stereo cartridge or a mono button on the preamp. The difference is not day and night - based on the stereo cutting process with a mono signal - but there is a clear difference. The mono button for example kills every life and is very boring to listen to.
A different chapter is old mono (no reissues). Here you have to go for a mono cartridge.