I mean, the sales speak for themselves. Gears of War could be considered a "killer app", like Nathan Drake or Zelda games, but Halo 3 was a global phenomenon.
As far as quality, both franchises redefined their respective genres, but I think Gears did a better job of staying true to itself over the years, and had a higher skillcap.
Like Gears as much as the next person, but I would say Halo, though both franchises slowly went downhill after their original trilogy was over and started having a different team work on them.
The recent updates on the Master Chief Collection on both current Xbox and PC is keeping Halo afloat at the moment (finally got Reach added to it too recently), but getting some players back through nostalgia aside, the series is still on thin ice, with Halo 6 Infinite being its last real chance to draw more people back in.