Well stereo is going to sound better because it's stereo meaning that the sound coming out of say Speaker 1 will be different than Speaker 2 which makes it more lifelike and makes the music more interesting sometimes. If they have a Mono set of The Beatles it's probably for the diehard fans who listened to them on records when they came out and love the old sound.
I heard fans saying that the mono version will sound better because the stereo version has an annoying compression on it. I also heard cuts from the stereo version. It does sound too compressed and just plain wrong. Also, I know that Beatles recorded mono once leaving no way to remix the tracks, so the stereo sound is an artificial process during remastering. I'd say go for the mono version; it's Beatles like we know them, only cleaned up but otherwise no other extreme adjustments made.
The Mono version also has a LOT less copies, and from what I've heard is already completely sold out at the distributor's level, so once they're gone from stores, that's it. If you're going for a collectible, that's it.
