You mean to prevent it? That depends on how you look at them. Nolva will directly block estrogen from binding to breast tissue. However, it inhibits IGF which is a bad thing for muscle growth.
Arimidex will inhibit the conversion test to est conversion. However, it's very powerful and very easy to inhibit estrogen beyond optimal levels.
As an antiestrogen to run from the start of a cycle, I'd choose arimidex in conservative doses. If I was attempting to combat early gyno symptoms, I'd go with the nolva.
(Note that this is in regard to ESTROGEN related gyno. Progesterone side effects are another story.)