I would strongly recommend full civvies. That said, half and half is good enough. The major concern of anything more than that being that some numpty may mistake you for someone in a position of authority and then you're in a REALLY bad situation. In an emergency, people will grab at the first uniform they see. Especially if you are wearing all black with a vest (be it body armour or LBV) etc...
Just so you have an idea, all that is required to be convicted of personate peace officer is to fail to correct another person's assumption that you are a PO. You don't have to read their minds, but if someone yells 'Officer! Help me!' you are required to state that you are not a PO as soon as reasonably possible.
Many of you know the differences in the uniforms etc., but most people don't care and don't pay attention and therefore have no idea that a guy in CADPAT or even woodland with an LBV or just a duty belt on isn't a PO and may have no clue how to help them. Immigrants from countries where the police wear military uniforms also add to the potential for mistaken identity.
Food for thought.