Boy, this is a tough question...
First, let us assume normally aspirated engines only (no turboes, no superchargers).
For a given displacement, diesels put out less horsepower then a gasoline engine. One reason for this is diesels operate at lower RPM then gasoline engines due to the higher stresses inherent in a compression engine versus an ignition engine (also, this is the reason diesel are built so much more robust then an equivalent gasoline engine). We could get into reciprocating masses, but I'm not competent to get into that (and the math required).
Diesels are also used more in commercial applications in which fuel consumption is more critical (i.e. costs).
However, torque, a more usable output of internal combustion engines, peaks at a lower RPM then an equivalent gas engine. Diesels also tend to be more of a constant RPM engine (higher efficiency), rather then varying up and down the RPM range as the way gas engines are used.
Bottom line, diesels are more robust due to being built to withstand higher compressions.
By the way, this is somewhat generalized. I'm sure someone can find instances in which diesels have a higher power output then a gasoline engine of the same displacement. But those would be the exception, rather then the rule.