And as far as their similar career paths:
Boy/October/War/Unforgettable Fire/JT/Rattle = Kill 'Em All/Ride The Lightning/Master/Justice/Black. The "essential early" albums. This is where both bands built their legendary, definitive sound and became insanely popular on an underground, and then populist level. All of these albums featured a new style, U2 with their spacey, ambient post-punk/stadium rock and Metallica with their groundbreaking form of heavy metal, dubbed thrash. Both bands would ascend to household-name status.
Achtung/Zooropa/Pop = Load/ReLoad/S&M. Here is where both bands, during the 90s, and with the help of some interesting Anton Corbijn photography, redefined both their sound and their image. U2 took their anthemic, straight ahead, spiritual stadium rock and made it electronic, dirty, and sexual. They made it dancey. They dressed in the latest trends and became 'cool' rock stars. Metallica took their epic, European, classically influenced metal and turned it into American roots music. Whereas they used to be influenced by classical music, this time they were influenced by the blues, country, and southern rock music. They made it bluesy.
ATYCLB/HTDAAB = St. Anger/Death Magnetic. This is where both bands, in the new millenium, became considered "legendary" in their time. Their peers were no longer The Killers or Slayer and were now The Rolling Stones and Black Sabbath. As a result, each band turned out two albums featuring a return to earlier styles of play, but not quite. ATYCLB was not a return of "80s" U2. They sound nothing alike. It's just that they both featured more straight ahead, spiritual rock songs. Similarly, St. Anger and Death Magnetic don't sound like old Metallica. But they are fast, heavy, and complex like old Metallica. This period features two albums which recapture the essence of the band and give a "we're back" statement. One album classic, the other heavily debated. In U2's case, ATYCLB is the classic while HTDAAB is debated. In Metallica's case, St. Anger is debated (and heavily thought to be the group's worst effort), while Death Magnetic is a classic.