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15mm all around is a great choice and you shouldn't experience any rubbing. 20mm for the rear would be flusher but you risk having some rubbing with a full car load. You can see pics of me on 350 pro-kit and 15mm/20mm here.
is it ok to have different spacer for AWD? since the distance between front and rear wheels are different?
if your wanting to use different spacer, you'd be better off to use the staggered rims combination...if you want to keep the awd format then use spacers no wider than 15mm all around.