You could always do both. But only have one army. Build your army in such a way that you can play it as either a loyalist or chaos force. Doing a cursed founding works best for this. Of course you may need to swap a few models or units out, but really you could use the same models for your basic marines. You could claim that they are either loyalists that have some taint but still fight for the emperor or that they are newly corrupted chaos forces that havent yet earned the favors of the chaos gods.