The problem with the newer Mac Pro, is the base model isn't really worth it because it won't be much faster. You're better off with a 5K iMac or whatever for the cost.
i think that might be true with the 6.1 and maybe next version.. once the nmp is 4-5yrs old, there will probably be a more noticeable gap in performance.. or, more software will be making full use of the 2 gpus and the imac will still have the one.. single core stuff though, they'll probably stay about even.