Direct Injection 2 stroke engines are more efficient than 4 stroke if driven exactly the same. The reality is they have much more torque and which uses fuel (but still more efficient) than a 4 stroke so the difference isn't huge but you will notice mid acceleration much quicker when on a steady state the 2 stroke will be using less fuel than 4.
I don't accept under any condition that the 2 stroke uses more fuel than the 4 stroke unless its producing more power, because by its injection system is so much more advanced.
The E-tec has a 3 year/300h service, not 500. Thats the G2 engines. BUT BUT BUT!!! that's a DEALER service you still have to do this annually on salt water:
Anti-corrosion anodes
Grease fittings, lubricate
Propeller shaft splines, inspect and lubricate
The manual recommends that annual checks should still be carried out as normal annually on everything else if in salt water such as pump impeller etc.
I'll tell you what you need to do, you REALLY REALLY need to get on a boat and test both engines. I suspect, but I may be wrong you will prefer the E-tec. I imagine that the dealer (and this is me being cynical) will make more money on the Mercury engine, but I think its quite inferior to the Mercury.
Finally
Ignore the American hype, it explains quite nicely the 2 stroke DI benefits....