Iowa in a walk.
1. The American armor scheme was the best ever devised, as was the armor itself. Kreigsmarine AP shells would have been decapped prior to penetration of the armor itself, and the internal armor scheme of the
Iowas was both unique and robust, with an interal armor around machinery spaces that was a great leap forward.
2.
Tirpitz, like her sister, was designed to fight a Jutland style battle, at Jutland style ranges. Her deck armor was not capable of defeating the USN 406mm AP shell (especially the 2,700 pound version) and her side armor belt, while strong, was too narrow to defend against long range plunging fire (part of the overall weakness of the German BB underwater protection flaws). POst war testing demonstrated the U.S. 406mm shell would defeat the best armor the Kreigsmarine ever produced.
3. Gunnery radar for the American ships was far superior to anything every mounted on any Axis ship. This is, perhaps even more than armor or shell design, the critical difference between the two ships. The Iowa hit what she was aiming at.
4. Screening for the U.S. ship, even without the presence of air support, would be far better than for any German BB. Not only would the U.S. ship have more CA in attendence than the Nazis ever had in operation at any one time, she would also have a couple of CL that had 155 guns that were both rapid fire and able to penetrate the armor of the German CA (just as the USN 203mm could defeat major portions of the
Tirpitz's deck armor at long range). The U.S. destroyers were also far better than the German counterparts, closer in capacity to the Kreigsmarine CL than their DDs.
There is a very nice anaysis of the merits of the different classes on the combinedfleet.com site.
http://www.combinedfleet.com/okun_biz.htm