Answered[ Edited ]
Well it depends....do you want a lot of everyday colors or do you want a combination of a few everyday colors and a few "playful" colors?
If you want lots of everyday colors go for the Naked Palette. If you want a combo then go for the Ammo Palette.
Something to also consider.....with the Naked Palette you get an eyeshadow primer which is fantastic if you dont already own one.
I own both of the palettes, and I can tell you from experience that in my opinion I think the Ammo Palette is much better than the Naked Palette because it gives you so many more color options. The colors in the Ammo Palette are paired together so you know which are the best matches to blend together, plus I love the different finishes of the shadows (from shimmery to micro-glitter). The Naked Palette is full of beautiful neutrals, but I personally very seldom wear neutrals (because I love color). You really need to think about your own personal taste when it comes to eyeshadow. Do you like having colorful eyes or brownish/neutral eyes? It really all comes down to each person's preference. My preference is the many various colors in the Ammo Palette. By the way, the quality of the shadow in both palettes is equally excellent Urban Decay's usual unbeatable quality.
I hope this helps you.