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Look at the price per 10' here https://www.elliottelectric.com/Products/Detail.aspx?c=14ALLTHREAD&v=BLII'd use 3/8" minimum. There probably isn't much difference in price between the two. If you're going into a minerallac strap, the hole most likely is sized for 3/8" rod anyway, and 3/8" rod is considerably less flexible than 1/4".
Look at Home Depot price