They are three miles apart and completely different. Doing both in a day works well if you take them in the right order.
Morning: Santa Monica
The pier early, before it fills. Then Palisades Park along the cliff, which is the best free view in the city. Third Street Promenade for coffee and lunch.
Afternoon: Venice
Walk or be driven down to Venice Beach. The boardwalk is chaotic and worth it. Abbot Kinney Boulevard, a few blocks inland, is where the good shops and restaurants are. The canals — genuine canals, with ducks — are quiet and almost nobody who visits the boardwalk finds them.
Evening
Sunset from the pier or from the Venice breakwater, then dinner on Abbot Kinney or Main Street.
The practical note
Beach parking in summer fills by ten in the morning and costs a small fortune. Being dropped at the pier and collected from Abbot Kinney removes the single most annoying part of the day.