russell,
As Sam mentioned, if you're coming from Switzerland then Stresa would be the obvious choice as you'll likely be travelling right past it on the line that goes through Domodossola. For the trip to Venice, it's only an hour or so from there to Milano Centrale and then an easy trip via Freccia high speed trains to Venice.
If you decide to visit Lago di Como you'll first travel to Milano Centrale and then take the Regionale to Varenna-Esino, which will also be about an hour (although you may have a short wait for the trains, as they only run about every two hours as I recall). On the way to Venice, another hour back to Milan and then the Freccia to Venice.
The travel time will be close in either case, except for the trip TO Lago di Como, which will add an extra hour or so to go to the lake (as opposed to being dropped right at the town on the way from Switzerland, which is the case with visiting Stresa. On the trip to Venice, the time is approximately the same. Note that your destination station in Venice will be Venezia Santa Lucia.