Dual core processors are more able to multi task and if not being asked to do two (or more) things at once will parallel to speed a single job.
I doubt there is any situation where a dual core 2.5GHz CPU would be slower than a 3.0 single core.
how much faster the 2.5Dual core CPU is depends largely on what specific dual core and single core
CPU is being discussed.
Most people doing demanding tasks aren't even using dual core CPU's anymore but rather quad (or more) core