Depends on what you're going to do. As noted, some apps cannot make use of all available cores, so faster CPU speed may make a difference. But it's all about end-to-end throughput: CPU, memory, disk I/O, and in some cases, graphics cards. If you're unaware of how "core-bound" you are now, then you probably want a faster CPU. macperformanceguide.com and Apps-Utilities-Activity Monitor are good places to start.