Fallout: New Vegas players are allowed to have one non-humanoid companion on their side. Is ED-E or Rex better?
ED-E is much better than Rex. The flying robot fires ranged projectiles and is a much better fighter than the cybernetic dog.
Rex is only recommended if you want a dog on your side for roleplay reasons.
ED-E or Rex | Acquiring the companion
Acquiring either ED-E or Rex is very different. ED-E is, most definitely, easier to get.
You run into ED-E at the Nash Residence in Primm. He’s laying on the counter, completely motionless. There are a few ways players can revive him.
The first option is to use a 65 Repair skill. If that’s too high, you could also use a 55 Science skill in combination with a 35 Repair skill.
Early-game players aren’t likely to have either skills this high. Your best option is to fix ED-E directly with two sensor modules, three pieces of scrap metal, and one bit of scrap electronics.
Players can find these objects pretty much anywhere. If for some reason you don’t have them on hand when you reach Primm, you can exit the town and head to Lone Wolf Radio.
The radio station is in between Primm and Goodsprings. If you fast travel to Goodsprings and run directly South from the map marker, you’ll find it. It contains all the parts you’ll need to repair ED-E.
This may seem like a lot of work, but it’s far less effort than acquiring Rex.
Rex can be obtained from The King in Freeside. It’s required you bring Rex to Jacobstown in order to have surgery done on Rex’s brain. He’s free to be your companion on the way there, and The King will give him to you after the procedure.
Considering Freeside is directly outside of New Vegas, players won’t be in this area until the Mid-Game. You’re likely to have ED-E before then, and you probably won’t want to swap him out by this point.
Combat effectiveness
In terms of combat, it’s not even close. ED-E has projectile weapons which can be used on any and all enemies. His combat music will alert you to threats, and he just acts as an excellent companion out in the wastes.
Rex is a melee-fighter for obvious reasons. He can only contribute by running directly into the line of fire. Although he can be upgraded when repairing his brain, it still does not make much difference.
We’d only recommend choosing Rex if you’re desperate to have a dog as your companion.
So that’s the answer. ED-E is much better than Rex.