in u2, when bono plays guitar, he's playing the rhythm part and edge is playing lead/solo parts. really though, many bands have only 1 guitarist and so they're a combo of the two...really.
which u2 song is hardest? well, of the ones i know i'd say hmtmkmkm is hardest cause of some of the repeated slides...of ones i don't know, i hear ialw is tough.
personally, i think rem guitar parts are often harder than u2. u2's all about effects/delay/distortion/ambience...that makes it difficult to get the actual sound right, but often the song itself is more simple.