re: First Prestige in COD:AW?

Use the permanent unlock on a gun or perk that you like to use but won't get until after level 30.

As for the loot, once you prestige you keep all of your loot, but will only have access to it once the weapon is unlocked. I'm pretty sure permanent loot is just random but it doesn't count against the 84 spots in the armory.

FWIW I took the IMR on my first prestige and Danger Close on my second. I will probably take the Exo Trophy system on my third since I tend to play objective based games like Domination and Hard Point.