Nike Structure 18 vs Nike Structure 20

I've actually found the 20 to have more support than the 18.

I wore about 6 pairs of the 17 and loved them, then 2 of the 18. I found the 18 to have less support and generally felt "floppier" for lack of better word. I now have a pair of the 20 and it feels reminiscent of the old 17's. As you probably know this is a more cushioned over-pronation shoe than others, so it still doesn't have that solid medial support. But I far prefer the 20. The upper material feels better too.

If you are under 160# woman or 200# man, get out of that shoe. Your foot should get stronger and be healthier in the long term in a lighter shoe. They "target" it to over-pronators just because it is hard to market to fat runners, but it is for fat runners.