Venice can be nice, but it rains there a lot this time of year. Also, the septic system on the entire island uses those picturesque canals not only as roads but also as sewer systems. You don't want to be walking around the residential areas in warm weather at low tide, if you get my drift. Venice is also very expensive. You can stay on the mainland at a hostel or campsite for very little though, if you want to rough it.
San Diego is a lot hotter and drier. This time of year it can be fairly moderate, around 70 during the day. That's probably a little too cold to go to the beach, but it should ahve a good night life, and Tijuana is just down the road.
From Holland Venice will be much cheaper for airfare, airfare to San Diego would prbably cost 800-1000 Euros.