Tandem!

The load is more evenly distributed as theres another axle. The tandem wiill have a lower center of gravity, far more stability in crosswinds, softer ride for the boat, and be easier to launch and load due to the lower profile. The only advantages for the single are a slightly lower initial cost, one less axle to maintain, and the ability to back around sharper corners if parking space is an issue.
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It's wonderful to be good. But it's better if you're lucky and good!