I've been thinking about this a lot the last year or two. Seen some here and there discussion of the subject on the site recently, so I am trying to pull it together in one spot.
For context, I'm thinking about the 24-28 foot size range, although maybe that should be moved up to as much as 35 feet?
A few things are obvious. Redundancy of twins is an advantage in getting home when something goes wrong. Singles should be lighter and have less drag with less running gear. Also, singles have lower centers of gravity, are more efficient I think (thust/gallon), and are physically easier to work on because of the space in the engine compartment.
By and large, singles are more likely to be higher-performance engines that will be more costly to maintain. (By this I mean more costly to maintain than ONE of the twins. It gets interesting when you try to sort out the maintenance costs of two smaller, lower-performance motors and their drives versus one larger, higher performance engine.)
I'm particularly interested in specific experiences. If a boat handled badly, what was the ill behavior? Etc, etc.