Like all combos, it depends on the percentage, character weight/floatiness and DI.
If they DI far away enough to the point where u-tilt won't reach, you can either opt for jump-cancelled u-smash or shffl'd u-airs--but be sure that you can reach them in time. If they're too close to the ground (i.e. they'll hit the ground and tech before you can land an u-smash/u-air) then you usually opt for the jump-cancelled grab.
Otherwise, if you're extremely worried that you can't catch them before they land, you can always wait for the tech/techroll and techchase that way.
Also, be aware that some characters love to f-air/sex kick right before they land to stop impending juggling (namely, Luigi, Marth, Peach, a few others). In these cases, you really want to think about whether or not you want to risk eating a sex kick in the face as opposed to continuing a combo. . .you can always dashdance away/wavedash back to evade the n-air and dash in again. Or other stuff like that, since this situation usually forces you to restart your combo options.