Nolvadex is commonly used to treat breast cancer. It binds to the receptors in breast tissue and blocks estrogen from binding to (and activating) them. This makes it great for preventing gyno but it's utility for other estrogen related sides like bloat and abdominal fat is limited.
Arimidex inhibits the aromatase enzyme which is responsible for aromatizing testosterone into estrogen. This prevents the production of estrogen in the first place. However, it is possible with too large a dose of arimidex to nearly shut down your estrogen production which can kill your sex drive and wreck your lipid (choloesterol) profiles. Without bloodwork you can't really know if you're taking the right amount.
Also, most people assume the arimidex is more costly to use since it's unit price is so high. However it is used at much lower dosages. 0.5mg/day for arimidex vs. 20mg/day for nolvadex (both typical dosages) are about the same cost per day.