Depends on what you intend to use the boat for and where you will use it. Cockpit/cabin room are always a commodity, especially if you will be boating with more than 3-4 friends or family. Kids? The extra room of the Sonic might be nice, but I have always been partial to Formula. I don't think you could go wrong with a Formula if it is in good condition.
Other factors to consider:
Fuel consumption of (3) outboards vs. (2) sterndrive
Can you get a seatrial under the circumstances in which you intend to use the boat?