If it's not high season, check the schedule. We needed to take an Alilaguna very very early in the morning to catch a flight, and the hotel informed us that the schedule had recently changed, and was running less often than we thought. We had to adjust our departure time from the hotel. Might be better for you because you are going from winter into spring.
I agree with the others' assessments of Alilaguna. It does the job, but is nothing special, and especially if it's dark out there is nothing to see from the low viewpoint. It isn't too cold because the large seating area is covered, but the front end is open for easy loading/unloading.
Because you will be on the Grand Canal it will probably be better to take a vaporetto to Piazzale Roma and catch the ATVO bus. That was a better experience. On the bus you sit in comfortable seats high off the ground (I think Alilaguna seats were waterproof hard plastic). The bus drives over the causeway to the mainland, so you can at least look out at the lagoon. However, Venice will naturally be behind you on that trip out of town, so you'll be facing the mainland. Last time I did it, I the bus was sort of racing alongside a train also heading for the mainland.
Lead times may be a factor. To catch the bus at Piazzale Roma, you do have to build in the time to get from your hotel to Piazzale Roma by vaporetto and on foot with your luggage, which could take a half an hour total depending on where you start. In some cases, a boat direct to/from the airport might be faster than vaporetto plus bus, when time is more important than money.