I've got a bootleg of Pepper in mono and there are some instruments and sounds effects that jump out at you more than on the stereo mix. George Martin has been quoted as saying, "You've never really heard Sgt Pepper until you've heard it in mono."
The Beatles were hardly present during the stereo mixing sessions, they pretty much just left that to the engineers, while they always attended the mono sessions. It wasn't until 8 track came out that they started to shift focus on stereo.
I listened to my vinyl mono copy of Sgt Pepper a couple of times at the weekend. I have to say that, in my opinion, the talk of the mono mixes being vastly superior is probably overstated. I'll leave aside the obvious point about vinyl sounding warmer than CD, and concentrate on the detail of the mixes.
George Martin gave a quote similar to the one above ("You've never really heard Sgt Pepper until you've heard it in mono") to Lewisohn in The Complete Beatles Recording Sessions. The book was first published in 1987, so Martin was most likely comparing the 1960s mono and stereo mixes, not the 1967 mono and 1987 stereo. That's an important distinction, I think, but it's since passed into Beatles lore that the 1960s monos are the best mixes out there.
Compared to the 1960s stereo mixes, sure, they spent more time perfecting the mono versions. However, the recordings were remixed by George Martin in the 1980s. The original mono mixes are certainly different from the 1980s stereo ones, but to say they're superior isn't necessarily true.
There *are* some things that are mixed differently in the original mono Pepper (Good Morning Good Morning sounds bassier and has far more punch to it, and the guitar licks on the title track sound somehow better). However, there are others elements that are mixed better on the CD version - instruments are often faded in and out with less jerkiness than in the mono, in particular. There are also details that were brought to the fore second time around, such as George Harrison's "three, four" count-in before the final verse of Within You Without You, which is much harder to hear on the original mix.
Either way it's all down to personal preference, and when they're all issued in September people will be able to pick and choose their favourite. I just wanted to point out that The Beatles in mono aren't intrinsically better; that's only the case when there are duff stereo versions to compare them to. Most people probably haven't ever *heard* the original 1960s stereo versions, as the majority of people had monophonic equipment and so bought the mono versions in greater quantities.