I had to say Maiden because Metallica were greatly influenced by Maiden when they started. Alot of Maiden songs are better than Metallica, but then again alot of Metallica is better than Maiden. Master of puppets, enough said.
I'm a bigger Metallica Fan than a Maiden fan, But Maiden were more influential, and also, THEY'RE BRITISH!!!
Very tough choice, but Maiden were the classics, they were there before.