To determine which kind of voting makes sense, you should decide whether you want to have agreement to many posts with weak consensus or whether you want to reward polarizing posts. In the former kind of system, “the incentive to downvote everything you don’t like to −1” is not a bug, it’s a feature. I’d also prefer a system with 3 extra buttons to an asymmetric one.
To determine which kind of voting makes sense, you should decide whether you want to have agreement to many posts with weak consensus or whether you want to reward polarizing posts. In the former kind of system, “the incentive to downvote everything you don’t like to −1” is not a bug, it’s a feature. I’d also prefer a system with 3 extra buttons to an asymmetric one.