I just did a scan over the 7 series datasheets to see their relative performance.  Here is a summary of the important figures; I figure they might be helpful for this and any other 7-series based projects:

Everything is for the -2 speed grade

The FIFO Fmax (block ram) has been a fairly good measure of the absolute maximum frequency we can expect to see out of hashing cores.  My rough estimates show that Artix 7 is likely to have a better MH/s/$ based on these figures alone, and single unit prices.  However, it is difficult to tell for sure, because I suspect that the Artix is crippled in some other way.  I have not check each chip's routing and CLB configurations.