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I think green is ok but if I could only have one I would pick gray.
I like grey better but it’s not what I like better but what the birds like better. I carry both and birds responded to the green slate and nothing else at a particular time. I can say the same about the grey. The first green slate I tried was in a cherry pot. Did not really care for it as it was way too high pitch for me. I have another in cedar and that thing is great. I believe a softer wood for a pot mellows it down. Not in volume but in the high pitch. Bottom line , get and hunt both. You never know what ole boy will want to hear at any given time.