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Rex's feats: shot down a cannon from 200 meters away at geonosis, reflexes are are faster then a commando droids and it seems easy for him to dodge blaster fire, he was strong enough to snap a commando droids neck, and knock out zygerrians with a single punch.
Han's feats: Has really fast trigger finger, reflexes (dodged greedos blaster shot at extremely close range) and fast gun draw.

Rex wins. He's basically Han with better combat equipment.

Rex kicks ass. He's no storm trooper