If you are going to be running a number of programs at the same time, or have a program that is written to take advantage of multiple CPUs, then you would be better off with the dual. Most programs nowadays aren't written to be split up onto multiple CPUs. So, while gaming, one CPU would prob'ly be sitting idle.
It might actually be worse than that... Sometimes you run into a ping-pong effect where the OS bounces the single program back and forth between the two CPUs, so it wastes processing time with each switch.
Unless you are running a server with multiple users, a single, really fast CPU would prob'ly be better.