In a pre-hersey thousand army which codex would you use ?
codex space marines or codex chaos space marines

Either really. For the ones I have in production I am using Codex: Space Marines (when the new one comes out since the army wont be ready prior to it) with Grey Knight terminator allies to be more specific. It depends on how much and what psychic type powers you want.

Both do work nicely but if you were to base them off of the Seige of Prospero the CSM codex may be a better choice for the sorcerer powers but you could get away with using either codex then too in my opinion.

It looks like the new Codex: Space Marines is going to have a lot of options for psychic powers, so it might be that this will be more truly reflective of the Thousand Sons at least during the Grest Crusade era.

However, I agree with Dreadnought Sagittarus, the Chaos codex might be better for the seige of Prospero and afterwards, although I would wait for the Tzeentch specific TS codex (if it comes out) as the current Chaos Marine codex is a steaming pile of horse poo.

"I am using Codex: Space Marines...with Grey Knight terminator allies to be more specific."

I know this will sound really geeky but grey knights didnt show up until after the Heresy so they wouldn't be able to in theory fight alongside pre heresy thousand sons

not really sure there was much point to that think i'll just shut up now lol

dude you do not sound geeky for one
this is what this whole forum is about
but i say grey knights are good
dont they have force wapons?
and they would be count as
... i think