Without getting into the general advantages/disadvantages of twins vs. single I'll speak to the specific reasons I think a twin Mainship 400 is a better boat than a single, at least for me.
1. Speed. The twin is capable of cruising at 15 knots at about 75 to 80% throttle. The single, from everything I read, is not.
2. Fuel burn. The twin burns only marginally more fuel at all speeds, and less at over 10 knots.
3. Draft. The draft on the single is around 6" more. That may not matter in some parts of the country but it matters in the Bahamas and Florida.
4. Engine access. Some on here are going to howl about this, but due to the layout of the ER Hatches, engine access on the 400T is the same as, if not better, on the twin. You open one hatch and you are down between the engines without moving any furniture. To access the port side of the engine on the single you have to move the couch.
5. Resale value. The twin engine boats are worth in the range of $10,000 more on the used market, and according to my broker, sell faster.
Again, there are advantages for twins in every boat (maneuverability, get home ability) and disadvantages (maintenance costs, unprotected props) but these are specific to the 400.