Boost grab is good for throwing at dash dances, and fake-outs on people who pay close attention to Sheik's dash attack animation (trick them into shielding, then grab). Most people aren't that attentive though, so its main, major use is just chucking into dash dance camping. Dash grab can also do this, and it's easier to space, but boost grab can do it too.
In the context of just running around and trying to space on the opponent's landing lag or whatever, I find JC grabs the better of the two because her body is more upright, the grab is higher (grabbing Fox out of his Nair is too good), and you're not as stretched out, so you're less likely to be hit when they're coming down, whereas the lower hitbox of dash grab will screw you from time to time when Fox or Falco or whatever tries to kick you.
Offensively, dash grab has some very useful properties (grabs inside her body more and away from her, more range, etc). So it's not a bad choice, but it's punishable with more frivolous things. But, quite honestly, the lag difference won't matter too much on the offense. If space animal sidesteps, you'll get shined either way.