I also voted for U2. And I TOTALLY expect this to be an absolutely dogfight to day 7.
Metallica is an amazing band. I have tried to reason through them in the past, claiming them to be "over rated." I even created a well traveled thread on why Megadeth was better than Metallica. But overall, you cannot deny the mark Metallica has made on the face of music in general. Doing what they did from a metal pulpit is all the more impressive.
Let's take the fact that U2's music, by design, appeals to a wider base of fans. We know that, but it doesn't make U2 a better band. As a whole, U2 has been far more consistent over their careers than Metallica has. We have not seen a decline in U2's music, not even for a day. Even the recent "How to Dismantle an Atomic Bomb" was a fantastic peice of musical work, and the hit song "Vertigo" was everywhere.
Metallica, on the other hand, has struggled to reclaim its glory days. Now, I personally thought Load was an amazing album, and one that truly showed their musical range. The backlash over Load was so ridiculous, and went to prove how short-sighted and often musically naive so many Metallica fans can be. But let's also face two major facts about Metallica - 1) S&M was a neat idea, but it was gimmicky. It was a repackage of old songs with new back up musicians. 2) St. Anger was total shit.
U2 also has a FAR greater international reach. While Metallica is huge statewide, U2 is legendary in the US, Ireland, UK, Canada, and Austrailia. As this tournament has taught us, it's one thing to be huge in one country (i.e.: Muse, Arctic Monkeys), it's an entirely other feat to transcend borders the way only a few bands - like The Beatles, Queen, U2, etc- have done. Metallica doesn't have the same level of claim for international fame as U2 does.
Let's also look at effect U2 has had on society. U2's music is so politicaly fueled, that the band and Bono have been asked on several occassions to meet with World Leaders and even serve as ambassadors in times of conflict. THAT is musical leadership. It's not like Metallica, whose biggest contribution to political society was the fight against music pirating. And Napster still made Lars Ullrich look like a clown.
Musical influence? Close, but I give the edge to U2.
Metallica has influenced a good number of successful nads, such as Korn, Godsmack (who were far more influenced by Alice in Chains than anyone else), Machine Head, Avenged Sevenfold, etc.
U2 is credited with the influence of Snow Patrol, The Fray, OneRepublic, Coldplay, The Academy Is..., The Killers, Your Vegas, Angels & Airwaves, Our Lady Peace, Mary J. Blige, The Chimes, Pearl Jam, James Blunt, tobyMac, Darlene Zschech, Pet Shop Boys, Ignite, The Smashing Pumpkins, Keane, Pillar, Utada Hikaru, Dream Theater, Sepultura, Saul Williams, The Living End, The Upper Room and The Bravery.
I went with U2 here, because I feel they are more musically complete, have more variety in their music, and have left a greater mark on music as a whole.
But I won't fault anyone for voting for Metallica here, either.