Abstinence! I greatly like and also enjoy both of these fantastic albums, for different and also personal reasons, so I'm sitting this one out along with LG.
I do remember watching a very interesting behind the scenes Beatles special on the tele several years ago with George Martin relating many stories about the songs, recording processes and outside influences on the band at the time of Abbey Road, which was very enlightening, imo. One of the more memorable things that I remember was with George Marton stating that one side of Abbey Road was mostly recorded for Paul McCartneys' preferences, and the other was mostly for John Lennon, save the other songs written by George Harrison and Ringo.
Dark Side of The Moon
Even though I'm a massive Floyd fan, DSOTM is not my favorite album just for the simple fact that it gets way overplayed. But when pitted against Abbey Road, The Floyd just takes precedence because of the sheer importance of Dark Side as a sonic revolution.
There are some of the best songs in the Beatles catalog on Abbey Road, but as a whole (which is how it should be listened to) Dark Side is the benchmark by which all other albums should be judged.