few of the biggest difference in skimmers is being able to correctly match air and water ratio and doing it in a manner that is most effective for that body style, use this as an example with one brand, euroreef use's the same pump and design on a few of their skimmers, now a sedra 3500 does x amount, but putting that pump on a body of x size and dimensions compared to their next model up, so is that pump really efficient on this or that one ?, most of the uppr line and even so now with the lower costing skimmers match a pump correctly to the body, a skimmer will only handle x amount of air to be at it's maximum effectiveness, some pumps pull a much better air to water ratio making them a more efficient pump to use, mojo on RC really gets into a more technical explanation