If the train connection was all on SBB and inside Switzerland, I'd take it over easyjet in a heartbeat. Coming from Venice over the Alps in winter is a different story (although I haven't done that route)

Does the train service you're considering require an advance reservation? If not, and the train leaves after the flight, then book easyjet, and if it doesn't fly, take the refund and get on the train. That's probably the best you can do short of traveling an additional day earlier