If you want to be a "purist" and go with what the Beatles intended, you go mono. Up to and including the White Album, the Beatles and George Martin (along with the engineer of course) would mix exclusively in mono. Stereo was new at the time and considered a gimmick, so serious time would be spent with the mono mixes, and the engineer (Geoff Emerick, Norman Smith) would have to task to put together a stereo mix in a couple days (though Emerick sure did a nice job with Pepper even though the stereo was just "tossed off".) I have both UK/Japanese mono and stereo pressings, and like both for differing reasons (yeah, not what you want to hear, I would say get BOTH.) Certain albums (mostly later ones) have very notable mix differences between the monos and stereos (and if you are a geek like me, you will want to hear both.) The best (besides if these see the light of day as vinyl, though don't know if they will top my UK Blue Box stereos and Japanese red monos) would be if they eventually sold the albums separately in mono and stereo. Then I would say get Pepper, The While Album Revolver and Rubber Soul in Mono.
Some differences: very different sounding "Tomorrow Never Knows" on Revolver. Srgt. Pepper sounds very different in mono, guitars are mixed louder on some songs, different sound effect levels, other things (John Lennon went on record as claiming mono Pepper to be the only version people should hear if they wanted what the Beatles intended.)
The biggest differences of all are apparent in the White album, and sometimes the mono mix is THE one for me (and sometimes it's the stereo mix.) Some songs sound quite different: Blackbird and Piggies have different animal sounds, Don't Pass Me By is at a different speed! But Helter Skelter....the mono mix is punk rock. Paul's voice is mixed dry and forward, the guitars are more raw sounding and in your face, and weird sound effects pop in and out more notably. Also, no false return ending and "I've got blisters on my fingers."
