But, it comes in two versions: mono e stereo. Why? What would you chose? Why the mono version costs more?

The Beatles were present during the mixing of the album in mono and the LP was originally released as such alongside a stereo mix prepared by Abbey Road engineers led by Geoff Emerick; the Beatles themselves did not attend the mixing of the stereo version. (The mono version is now out of print on vinyl, but was re-released on CD as part of the Beatles in Mono box set on 9 September 2009 worldwide) The two mixes are fundamentally different. For example, the stereo mix of "She's Leaving Home" was mixed at a slower speed than the original recording and therefore plays at a slower tempo and at a lower pitch than the original recording. Conversely, the mono version of "Lucy in the Sky with Diamonds" is considerably slower than the stereo version and features much heavier flanging and reverb effects. McCartney's yelling voice in the coda section of "Sgt. Pepper (Reprise)" (just before the segue into "A Day in the Life") can plainly be heard in the mono version, but is nearly inaudible in the stereo version. The mono version of the song also features drums that open with much more presence and force, as they are turned well up in the mix. Also in the stereo mix, the famous segue at the end of "Good Morning Good Morning" (the chicken-clucking sound which becomes a guitar noise) is timed differently and a crowd noise tape comes in later during the intro to "Sgt. Pepper (Reprise)".