Q article/RS article
Moment of Surrender was described as being "created" during the current album sessions off of some chord changes that Eno suggested, which the band came together and just started playing to. In no time the song was born and the band recorded it in ONE take. It has a syncopated beat and a grand swell or something to that nature.
Mercy was created in prior sessions for HTDAAB and maybe even ATYCLB, it was worked on for a while and to my knowledge was not recorded in one take. It also does NOT have a syncopated beat, its actually more of a constant beat that doesn't change too much from beginning to end.
Not to mention the fact that everyone thinks MOS is one of the top two songs on the album....as opposed to a "glorified b-side".
The only similarities they may have is that they are longer than usual songs and U2 wrote them both.
Therefore Moment of Surrender is NOT Mercy.
I rest my case