Pivot grab stops you in place, while jump cancel grab keeps you sliding during jumpsquat.
Also, if you want to grab behind you from a dash then pivot grab is at least 1 frame faster than jump cancel grab; you grab on frame after the pivot, as opposed to entering jumpsquat on frame after the pivot and then grabbing on the frame after that (at minimum).
Watch a high level Marth fishing for grabs in neutral, and look for the distinction between pivot grab and jump cancel grab.
You can pseudo-option select between pivot grab and jump cancel grab by inputting a jump on the same frame that you turn, then inputting the grab on the frame after. This way, if you execute the grab too late then you'll get a jump cancel grab (which is better the dash grab you'd otherwise get), but if you execute the grab with the correct timing then you'll get a perfect pivot grab. It makes you potential technical flub less costly with no downside.