While not as cinematic as Call of Duty or Battlefield, Gameloft's Modern Combat series does a decent job at keeping up with the big boys. You'll shoot your way through a variety of set pieces in the campaign mode, then take your skills online to show off how good you are at getting headshots. There is even DLC in the form of extra multiplayer map packs.
One of the best third-person shooters on the iPad, Mad Finger's Shadow Gun has all the makings of an epic. You'll explore huge levels, fight your way past waves after waves of bad guys, and take down massive bosses. The latest update added an expansion pack, giving you another adventure once you finish the main storyline. No multiplayer in the iPad version quite yet, but chances are we'll see it in a future update.