Go with the 15 and trolling plate. I upgraded my 14 ft Jon boat several years ago and loved it! The 15 will give you more control in rough water, plus get you to your spot quicker which is always a plus.
The drift sock is a bad idea. I tried that and was sorely disappointed. Drift socks are great for slowing rigs down in the wind, but trying to pull it through the water under power at anything over 1 mph caused me great grief.
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