With a trip planned for March, not normally a peak period, you have a very good chance of scoring a good deal by watching fares diligently between now and December. Decide what would be a great fare on an itinerary with as few stops as possible and grab it if you see it.
I echo the others in saying that price and elapsed travel time are far more important to me than the airline, though I stick to major carriers, avoiding the European equivalent of our US commuter airlines. Flying overnight is a pretty miserable experience; flying for 15 hours overnight is considerably worse than flying for 12 hours.
It might be worth your while to watch prices into Milan as well as into Venice. From some origins Milan commonly runs a lot cheaper, and you may find (I haven't checked) that you can get to Milan with one less stop than will be required for Venice.
I recommend avoiding any itinerary that will require separate tickets going or coming. That sort of thing leaves you vulnerable if there's a delay on the first leg, because if you miss flight #2, you'll be buying--and paying for--tickets on a replacement flight. I would only cobble together a multi-ticket itinerary if I saved a great deal of money (perhaps $500 or so per person), because for anything else I would consider the risk not worth it.
Edited to add: The conventional wisdom seems to be that Paris CDG and London LHR are harder to change planes at than Frankfurt, Amsterdam and Dublin. I have consciously avoided tight commections at CDG and LHR because of those warnings, but I wouldn't pay a lot of money or add 4 hours to my travel time to avoid them.