Stir fry also is synonymous with wok cookery, and a true honest to goodness strit fry IS done via wok.
The hand hammered woks are the best bang for the buck, last generations, and really handle a LOT of cooking requirements. Super thin bottom for the best/hottest heat, tapered sides with increasing thickness for parking items, different temp zones, a good wok(and a good cleaver)can be all some folks need for a set up.
Saute is more of an in-place, slightly tossed type oh high heat searing, whereas stir fry is an active, things are getting tossed around, fast, super fast kinda thing.