If you aren’t installing it yourself then a used unit isn’t even gonna save you all that much money unless your buying a higher mileage one which isn’t ideal, also when your dealing with a Trans that has known defects from the factory that make it prone to failure it’s better to get a rebuild so they can correct these known issues whereas with a used unit your at risk for repeated failure.

I’d investigate any Potential Valve Body failures before deciding the transmission is 100% shot, I’ve seen so many cases where a transmission was diagnosed as failed even by TRANS SHOPS who specializes with Transmissions, but it was just a Valve body.

One case I actually had a Transmission in an 08 impala, severe shifting issues over a year even after fresh fluid, was nothing but a clogged shift Solenoid threw some seafoam in, and it went away then changed the Fluid zero issues many miles since, the transmission shop told me 100% needs a rebuild before this.

Regardless, I’d take the rebuild especially if they offer a good warranty.