Cheaper just to choose one. Looks like LX150 is probably the best bet, but would be nice to have some compilation results.
I had a look into the documentation of both the Spartan 6 and the Cyclone IV series.
Xilinx SP 6 :http://www.xilinx.com/support/documentation/data_sheets/ds162.pdf
Altera C IV :http://www.altera.com/literature/hb/cyclone-iv/cyiv-53001.pdf
As far as i see they both require a 2.5 V and a 1.2 V rail.So the only difference would be how much current each rail needs.(
please someone verify that fact)
The EPE power calculation gives the ability to estimate the power needed for each rail.
Edit: Also, after a slightly tedious 4-hour build process, Fmax=109.29MHz and 97% resource usage for the fully-unrolled DE2_115_makomk_mod on the EP4CE75F29C7. Might be able to get it up to 110MHz with the right options, but I wouldn't bet on it.
So i see it as a given it is possible to run a full miner core on the Altera cyclone IV 75k. Therefore it shall be one final canidate.For increased performance lateron we might have a look into the Cyclone IV GX 150k device. Maybe someone could run a compilation for this one. Also i checked the website of altera and found the FPGA'S in their online shop to be cheaper than at digikey in some cases.This might improve economy.
If the power supply of the Spartan 6 series and the Cyclone IV turn out to be really interchangeable i see no reason not to try a prototype for both based on the same board.