I look at the chart postitions (Mostly the US Bilboard)
U2:
http://en.wikipedia.org/wiki/I_Will_Follow
http://en.wikipedia.org/wiki/New_Year%27s_Day_(song)#Chart_positions
http://en.wikipedia.org/wiki/Sunday_Bloody_Sunday_(song)
http://en.wikipedia.org/wiki/Pride_(In_the_Name_of_Love)
http://en.wikipedia.org/wiki/With_or_Without_You
http://en.wikipedia.org/wiki/I_Still_Haven%27t_Found_What_I%27m_Looking_For
http://en.wikipedia.org/wiki/Where_the_Streets_Have_No_Name
http://en.wikipedia.org/wiki/Desire_(U2_song)
http://en.wikipedia.org/wiki/All_I_Want_Is_You_(U2_song)
Coldplay:
http://en.wikipedia.org/wiki/Shiver_(Coldplay_song)
http://en.wikipedia.org/wiki/Yellow_(song)#Charts
http://en.wikipedia.org/wiki/Trouble_(Coldplay_song)#Chart
http://en.wikipedia.org/wiki/In_My_Place
http://en.wikipedia.org/wiki/The_Scientist_(song)#Charts
http://en.wikipedia.org/wiki/Speed_of_Sound_(song)#Chart_performance
http://en.wikipedia.org/wiki/Fix_You#Charts
http://en.wikipedia.org/wiki/Talk_(song)#Charts
http://en.wikipedia.org/wiki/Violet_Hill_(song)
http://en.wikipedia.org/wiki/Viva_la_Vida
http://en.wikipedia.org/wiki/Lost!
(I think it's clear that U2's singles did better) I also hear all of those songs on the radio ALL OF THE TIME on different radio stations, so that tells me that they were pretty big singles.
U2 doesn't have as many singles as Coldplay, but the singles they did have were really successful. Coldplay has a lot of great music, but the singles didn't do AS well as U2.
It's safe to say, though, that competition in the music market has changed substantially over the past 30 years and there have been many other great artists other than Coldplay this past decade (In my personal opinion Coldplay is the best).
It was only when U2 released Joshua did they beat their competition by a large margin. Joshua Tree (Their fifth album) happened to be their breakout album which catapulted them into superstardom. They only cemented their status as rock legends when they released their seventh album (Achtung Baby... in their second decade) because Rattle & Hum didn't do as well.
Coldplay's breakout album is hard to define... Yellow was a huge single, but I still have to say that their second album was their best album.
Coldplay also hasn't had a flop. U2 had a flop in their first decade. That was the album 'October'.
Listen, I'm not saying one band is better than the other. I'm only trying to prove they're both great bands and they both respect eachother.
And SERIOUSLY?!?! There is this accusatory copy-cat allegation that Coldplay has had for a LONG TIME! Personally, I see a huge difference, but even if there were striking similarities here's this example: The Kinks and The Beatles sound alike; Led Zeppelin and AC/DC sound alike; Soundgarden, Creed, Stone Temple Pilots, and Nickelback all sound alike!
It's simply just that people don't like Coldplay which is OKAY. But THEN they have to go further to prove their point and they come up with BULL like these arguments.
In conclusion to this... rant? I would like to repeat they're both great, and if you don't like Coldplay that's fine! but you need to get over it.