What are you primarily planning to do with this boat?
Take the family out for day trips?
300 mile roundtrips over the weekend, staying on the boat?
If you are buying the 310SS, get the 496HOs. You can't have too much HP. New Formulas come with a factory 5 year warranty. If you are only keeping it 3 years or so, anything that goes wrong in that engine compartment is their problem; not yours. The only thing you will be responsible for is impellers, oil changes, and spark plugs, as long as you don't screw anything up.
If you can swing the bigger boat (payment, insurance, fuel, logistics) now, do it now. You are going to be ready for an upgrade before you know it. Four-foot-itis sets in quickly, and its a tough one to shake. Seriously, if you can handle it, don't be intimidated by the bigger boat. It only takes a little while to get the hang of your new boat.