Undoubtedly for a first timer, New Digha Beach should be the top priority, particularly if you are planning to go inside the sea (and that's of course :) ). Bathing in Old Digha has been illegal now-a-days and it's risky as well. Better to stay in New Digha, and drop by at Old Digha any time in the afternoon or evening.
Please enjoy freshly prepared fried fish and jhalmuri at Old Digha sea shore with evening breeze. (I hope you are non-vegetarian :P )