To answer your question, rather than give unsolicited advice about how many cores to get, I am planning a 12-core purchase and am leaning towards the cheaper clock option as it will save a lot of money for other bits and pieces.
I will run GridMathematica, which can use all 12 cores very effectively - in fact the question in my head is whether I can fire up just 12 kernels or go for the full 24 using the hyperthreading. Having found that a quad core Windows laptop (with the proper i7-720) can fire up 8 kernels and make good use of them I am now wondering if the big mac will do 24.
I think going for the 12 is sensible for future proofing. My 8-core 2008 Pro is still doing fantastically well running 8 kernels, and I have never regretted going for that over the four core nearly 3 years ago. Then again, I always knew I could use all 8 cores to the full.