Nolva and clomid are serm's, they bind with the estrogen receptor preventing estrogen from interacting with it. Aromatase inhibitors (AI) like arimidex and aromasin prevent the conversion of certain AAS to estrogen. I believe aromasin is "safer" in that it affects the lipid profile less, and it also increases igf-1 levels which can help growth and help maintain gains during pct. Arimidex is cheaper and has a longer half life so it can be dosed less frequently. Letrozole is the strongest of all and can lower estrogen too much as well as having a greater negative effect on the lipid profile. Letro is best reserved for reducing gyno, rather than on cycle/pct.
On cycle an AI is the preferred method of controlling estrogen, serm's should be relegated to backup gyno control and pct, using nolva and clomid respectively.
Unless you're referring to other serm's like toremifene/raloxifene, of which I know little about. As far as AI's go it's probably Aromasin > Arimidex > Letrozole