Both of those people would be equally unacceptable as a professional translator. It's true that #2 would be better if those were your only two options, but that's an unrealistic limitation.

A guy who is 99% fluent in source and 100% fluent in target will generally produce a flawless translation--if he happens across a word or idiom in the source that he doesn't quite understand, it will stand out to him, and he will pause to research it before continuing.

A guy who is 100% fluent in source and 99% fluent in target will produce a perfectly clear but very slightly stilted translation, because he's not aware of the few mistakes that he makes.

If you're translating a commercial product and you have my two guys to choose between, you would want #1.