2013 Mac Pro: Lead Times Extend to March at Apple Store, but B&H has the 6-core in stock
Authorized Mac dealer B&H Photo has the 6-core 3.5 GHz Mac Pro in stock with free shipping and free Parallels Desktop 9.
B&H Photo also has substantial savings on AppleCare versus the Apple price.
The 6-core is the “sweet spot” model for most uses and users as the tests show. The choice of SSD (flash size) speaks to particular needs (I prefer 1TB, but 256GB is ample for many uses).