I have new durarock and green board on my walls and floor on the outside corners should I put corner bead and mud before tiling or should I use thinset

Is this a bathroom installation? I need to ask you to restate your question, maybe using punctuation so I can picture what you are doing. Greenboard on the walls in a bath is no longer the standard if you are inside a shower or bath tub.

Use mud. Thinset will be very hard to sand and smooth out. Just try to keep the area where the tile will be set clean.