Your question is not vague enough! You're supposed to say "which processor is more efficient" and stop there!
Kidding
Going out on a limb and assuming by efficient you're talking in terms of power consumption. In which case, less is more (more as in better). Fewer cores will require less power which leads to better efficiency. For most applications, a CPU with 2 cores will suffice, and require less power than a comparable quad-core (clock-for-clock). However, in terms of overall performance (assuming you're running multi-threaded tasks and/or Windows 7), more cores will get the job done quicker, generally speaking.