I'd use eccentric where I want the bottom of my pipe to remain at the same elevation to allow drainage into the smaller diameter piping, or to allow a standard support (ie. pipe shoes) to be used on either side of the reducer. Otherwise I'd use a concentric.
Eccentric reducers are also almost always used at a pump suction, flat side up. The rationale is that if you have any gases in the suction liquid, they won't tend to 'collect' in the pocket a concentric reducer would create at the top of the pipe. A pocket of gas so created would get carried into the suction as 'slugs' which is not good. With an eccentric reducer, any gas bubbles would continually be carried into the suction in small amounts (pumps can handle small amounts of gas though no gas is best).
For piping, reichertc is on the money. I would say if you don't have a good reason to use an eccentric reducer, use a concentric.
Horizontal steam lines with a forward slope require eccentric reducers. Concentrics allow condensate pockets to collect. Concentrics can be used on vertical steam runs.
