Get Boston.
Bono's voice was much better in Boston.
Larry screwed up the drum entrance to Angel Of Harlem in Slane.
Bad is a better live song that All I Want Is You. Bono also screws up the sanctity and beauty of the song by murdering it and singing "all I want is fucking you." He had no such hiccups in Boston.
Stay in Boston was much better than the spiritless performance of Staring At The Sun in Slane.
The Elevation Tour was an indoor tour for the songs off the All That You Can't Leave Behind album. Slane is not representative of that tour as it is outdoors and omits key songs off that album - New York and In A Little While.
Slane has a shorter setlist filled with "fillers" of old tunes that have been done too many times in the past and where you can get better versions of those songs from previous tours.
Bono talks too much during the Slane DVD. We're buying the DVD to watch U2 perform, not to hear Bono sing.
Slane features a much longer version of one of the most unbearable songs off the POP album - Wake Up Dead Man. At least the version in Boston is shorter so the agony isn't as bad.
Boston has better video and audio quality, more camera angles, and basically a better production in general.
So I think it's a no brainer, get Boston.
Cheers,
J