Is the Reaper than Rexes really better at Outposts and other missions on Lost Colony?

So far I could only do Gamma defenses with my rexes that range between lvl 280-300s. But three of my rexes got demolished on one outpost attack mission, all of which had above 30K HP (no saddles though so not that strange I guess). I never dared to go back with the rest of my rexes since I know I won't win. Is it better to get a good rex saddle bp for my whole army or use a reaper and use the rexes as an army?