I'd prefer the higher velocity, as noted several times including in Seth's OP, it allows for a larger margin of error. However, watching games, it seems to me most pitchers get their strikeouts on breaking balls. Kyle Gibson has fine velocity, but he relies on his sinker quite a bit, and sinkerball pitchers don't tend to get a lot of strikeouts. Funny thing is that Fangraphs rates Gibson's slider and changeup as easily his best pitches, it would be nice to see him whip those out more in two strike counts and see if the results change.