I'm not really sure why people feel the need to tie themselves exclusively to one particular method.
Both when boxing and soloing (my main is a raid healer) I swap fairly smoothly between the methods as needed. You can't strafe on the mouse, for a start, and if you're trying to pick up a target with the mouse but need to keep moving, you're going to struggle if you're only used to mouse movement.
The "best", then, is to be proficient at both, whether for PvE or PvP

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