What are the advantages and disadvantages of low vs high amps and watts?

I want to get a car charger for my IPhone 6 (not 6s). I see different options for electrical amps and watts. What are the technical reasons to choose one over the other?

Belkin Car Charger (2.4A/12W) with Detachable Lightning Cable

Belkin Car Charger (2.4A/12W) with Detachable Lightning Cable

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