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Ask your girlfriend bigger is better. No kidding, you will always get a better ride out of a bigger boat of the same type. But bigger also means bigger HP, bigger trailer, bigger truck, bigger storage, more fuel use, etc., etc. It is like riding a small compact with shorter wheel base at 35 mpg compared to a longer wheel base full size sedan at 18 mpg.
You must weigh each factor to see if you want to up-size.