Lars openly cites the NWOBHM as the reason he got into music as opposed to playing tennis. Therefore that would make Maiden the more influential of the 2 groups because Metallica wouldn't have existed in their original form without the influence of Maiden.
Both put on great live stage shows but are totally different. Maiden means you dig out your denim shirt covered in patches and throw the horns all night having a party. Metallica means dressing in a black t-shirt, getting ****faced and bruised to hell in the moshpit.
