I think they are similar in that when you listen to these two bands, you can tell that they write songs that will the back wall of the largest stadiums just as hard as they hit the front of the stage. They write HUGE songs. Even the smaller tunes are quite ridiculously large.
They have their obvious differences, but if you take a step back and look at the larger musical picture, you can see lots of easy comparisons.
Having said that, I think U2 is struggling. "I'll go crazy if I don't go crazy tonight"? 'Nough said.
Meanwhile Coldplay has hit their stride with VIVA finding a different way to be a band. I can't wait to see what Coldplay comes out with next while I could probably care less about what U2's next move is.
But, in the words of Dennis Miller, "Of course, that's just my opinion, I could be wrong."