For jazz through the late '60s, you want mono. For classical you generally want stereo where available. (Not in the case of certain French labels from the '50s and '60s.) For rock it's partly a matter of taste, partly a matter of how important which mix was to the group in question. For the Beatles, for instance, you definitely want mono through Revolver or so; then you want both through perhaps the White Album. Late '60s rock mono records are generally curiosities made in small quantities, and can go for lots of money - the Kinks 'Arthur,' etc.
A true stereo mix cannot be created from a mono master if there are no multitracks. Fake stereo re-creations were made in the '60s in many instances and they are monstrosities.
A true stereo mix cannot be created from a mono master if there are no multitracks. Fake stereo re-creations were made in the '60s in many instances and they are monstrosities.