First base your decision on the needs of the applications that are most important to you. Applications that will benefit from 10 or more cores a few - just make sure that your applications fall into that category. Otherwise, it could be a waste of your hard earned $$$.
Secondly, if your needs aren't immediate, why don't you wait for a nicely discounted 4-core 2013 MacPro refurb to appear, then buy it.
I'd then wait for the price of one of these E5-2687W v2s [
http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon E5-2687W v2.html ] to drop below the current price of [ $2200 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116937 ]. Eight cores running steadily at turbo stage 1 which is 3.6 GHz [
http://www.xtremesystems.org/forums...iF-and-Ivy-Bridge-xeons&p=5206937#post5206937 ] is nothing to sneeze at, and with a max turbo boost to 4 Ghz, well need I say more (other than to point out the 150 W TDP).
PS - Then, and only if needed, I'd add a quiet circular fan [ from here -
http://www.directron.com/quietfans.html ] to the top to pull in more cool air from the bottom and to exhaust the hot air from the top sooner. Maintaining the coolest system possible helps in keeping turbo stage 1 almost constant.