Your decision should be based on what is the cheapest flight to Dublin out of Venice -Aer Lingus Venice/Treviso- Ryanair Milan-Linate Milan-Malpensa (which is a long way westward past Milan). Actually, as I'm unsure whether Aer Lingus still fly from both milan airports-check it out.
Also, check the times -some flights depart quite early. Between leaving hotel and flight-time you need four hours, maybe longer in case of traffic.
Also does the car-hire company (i assume you're renting) have facilities at each airport.
Personally I find traffic round Milan -on the autostrada - can be horrible much of the day.