Disagree

Hobby isn't near downtown per se. It's more SE between 610 and the Tollway.

Either is probably fine to fly into, as Katy is due west of the city, west of the Beltway/Tollway and halfway between the north end of the 610 loop/Beltway and the south end of the 610 loop/beltway concentric circles.

Realistically, Hobby is probably closest from a pure mileage standpoint, but both Hobby and IAH are going to dump you onto the Beltway (although I'd pay the tolls), and loop you around, then dump you onto 10 near Memorial, then it's West to Katy.

That said I haven't driven a ton in Houston lately, and all that driving has been confined to primarily the NW side, so if there is Tollway construction, that would change my answer. But IAH is more north of Katy than Hobby is South of Katy, and at that point, you're just using the outer rings to get to 10 then West, although depending on where in Katy, the Westpark Toll could be an option as well.