Is it better to have a 1.5 GHz dual core processor or a 3.0 GHz?

the 1.5 dual core can handle 2 threads at the same time and 3.0 single core can only handle 1 thread both of them switch threads rapidly if you have CPU-z you will find that the only difference in number of threads (the number of cores in single core is 2)

short answer : the dual core is faster in multitasks

the single core is faster in single task (like most of games)