I switched to 16's a couple of years ago. 98% of the time, the 12's worked just fine. The advantages to the 16's are when fishing real shallow or very clear water. The extra length gets the baits further from the boat to keep from spooking fish as bad. Plus, as Feelay said, you do get a wider spread with the longer poles. If the fish are deeper and/or the water is dirty, the pole length doesnt really matter as much.

