Yeah, it's pretty much if you want to take the risk. Tri Attack/Hyper Beam is your main attack, so raising those is a priority over the other moves. But if you'd prefer to wait, and bring it in on a physical wall (or just anything with higher Def), go ahead and use Download. I wouldnt though, because PorygonZ is enough of a threat to make things like Blissey switch out, but you aren't switching into Blissey if you have Download.
But Adaptability is definitely better if you're using a Mixed Sweeper.
Of course, the last problem with PZ's ability, is the fact there's only 1 Dubious Disk in the game, so if you're like me and imported a Porygon2 from R/S/E, you're stuck with what the game gives you. I'm very happy with Download, but like I said, it's all about personal preferances, and of course, what the rest of your team is like.