The pros of twins:
They are easier to dock and maneuver in tight spots, which is nice if you have to deal with wind and currents. You have the insurance of a second engine if you have engine trouble, possibly saving you from having to pay an outrageous towing bill. They're necessary to get any performance out of a bigger boat, say 30'+. You can powerload twice as fast (haha).
Cons:
Additional cost of fuel, double cost of engine maintenance, they're heavier, they'll usually give you a lot less room to work on them (PITA if you do your own maintenance, possibly more costly if you have someone else do it), less (or no) storage space in engine room, could double the cost of repair items (think 2 drives/props/couplers,etc, when you hit shallow water…

An example of higher costs is the no-drip trim pump caps. They seem like a no brainer at $7.95 per, but I need 4, oil changes require 16 qts, prop reconditioning X 2, etc.
I've got twins in a true 27' boat (28'11" loa) and a single was never offered in it. Formula replaced this model with a 27'1" loa model and only sold it with a single. I'll probably always have twins because my next boat will probably be bigger and I'm already about at the cutoff for single engine apps. Engine access absolutely sucks, fuel costs are higher, but I try to do as much of my own maintenance to save there. If I were buying new (which I won't) in the same size I have now, I'd probably go with a big single. The newer stepped hulls are more efficient and you shouldn't have to worry too much about engine troubles with a new boat under warranty. I'd definitely buy unlimited towing insurance though.