The train between San Antonio and Los Angeles only runs 3 days a week. It is the Sunset Limited (#1). The Texas Eagle has 2 cars, a coach & a sleeper, that come off train #21 in San Antonio and is attached to the back of the Sunset Limited (#1) on those days the Sunset Limited runs. Those 2 cars show as train #421 on the booking page.
It is mostly, if not entirely, desert scenery between San Antonio & Los Angeles.
The Seattle to Los Angeles train, the Coast Starlight (#11) runs through mountains and along the coast.
It depends on what you want to see.