Ployprop fittings are usually thicker than copper ones. That means for the same OD poly fittings have a smaller ID.
I've never cracked polyprop fittings even with overtightened metal clamps. Also tygon tubing sticks so well on those, clamps are not needed.
With copper fittings you'll need clamps everytime. But better flow and "round" elbows make them *quite* interesting.
If i had the choice i'd go with slightly oversized copper fittings for the better flow.
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