For many of the processes in the program (modeling and much, but not all, of Simulation) the program is single threaded. For that reason you usually want to go with the absolute fastest clock speed you can find for a CPU (and a lot of people overclock as well for this reason). There are only a couple places were multiple cores are a noticeable benefit over clock speed; these are the Flow Simulation solver and PhotoView 360 rendering. I'd only recommend the 8-core CPU over the 4-core CPU if you machine is going to be used primarily for Flow Simulation or PhotoView 360. Otherwise, you are going to see much more benefit to your daily modeling tasks from the faster CPU.