I was late to loving this one, but if you're used to the stereo one and adore it, you're missing out big-time by not picking up a mono copy of it on vinyl.
There is a ton less reverb on his vocal, and Bill Miller's piano is massively higher in the mixes for most of the LP.
This gives it an intimacy that is much different from the 'grand-stage' sort of sonic image the stereo album has, and the piano being so high in the mixes gives the album a sense of song-to-song continuity that is comparatively lacking in the stereo mixes.
It was more likely to be originally heard in stereo by Sinatra's higher-income adult listeners than pop/rock'n'roll records which the teeny-boppers would have played on their own cheaper equipment. Nonetheless, stereo was still a novelty in the 1950s' so this is probably how the average listener would have heard it on it's initial release. I can imagine what a shock it was to pick up the CD in the 1980s and hear these 'huge' ultra-dramatic mixes on the stereo version. You'll have the same shock in reverse when you first hear the mono version.
As a long-term British Invasion brat, I find it quite ironic that the stereo mixes are the heavy-reverb mixes, whereas a lot of stereo Capitol Beatles albums were treated to a massive amount of reverb to make the records 'wetter', and then the mono mixes on albums like 'Beatles '65' were fold-downs (this is where you take the stereo mix and combine it onto 1 channel for mono) of these hideous sonic crimes.
If you think 'Duophonic' is bad, you ain't heard nothin' yet. At least duophonic was a misguided attempt at making stereo out of mono. These US mono mixes of Beatles tracks were post-production destruction jobs on tracks that already had stereo mixes, a destruction job made that much worse by then combining the two channels of sonic muck into one big pile of 'wet' mud.
If you're interested in these sort of things and have the Beatles Capitol Albums box set, take a listen to 'She's A Woman' and 'I Feel Fine' on the mono 'Beatles '65' and you will rarely hear a bigger pile of junk. These were the mixes that were used on the US single of those two as well. Then pull out Past Masters and prepare to be shocked at how 'dry' and clean the UK mixes were.
Apparently contemporary US kiddies were thought to be more attracted to the louder and more abrasive 'wet' mixes, whereas UK kiddies wanted to actually hear the instruments.