I have a 6s that needs a new battery. Should I get a high capacity 3200 mah replacement?

You should ONLY get your battery replaced at an Apple Store or Apple Authorized Service provider. Comes with a warranty, is authentic (which you cannot get anywhere else). $49 (in the US).

This link will get you on the right track to schedule a battery replacement --> iPhone Battery Replacement - Official Apple Support

kimradco ~ You should get a genuine iPhone battery. Capacity should not be an option:

About genuine iPhone batteries

...found by searching Apple's Knowledge Base.

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