With the FPGA Arcade Replay Board on the verge of becoming mass produced (compared to other projects in the Amiga market only), it probably has enough room in its FPGA to contain the SAGA code and could double or quadruple the number of computers that can run SAGA capable programs in the future, since the Natami will be a higher spec and higher priced product that will not match the popularity and wide spread distribution of the Replay board.
By my guestimates, I'd say the Replay board won't have enough room for the SAGA chipset alone, much less the SAGA and the N050. Also, the SuperAGA chipset is being designed in AHDL so it will only work on Altera FPGAs without conversion to VHDL. The FPGA used in the NatAmi will have approximately 4+ times the capacity of the Xilinx one used in the Replay board. Of course that will cost more than the Replay board, but it will also be more future-proof. Just because the NatAmi is still in the alpha-test stages doesn't mean that it won't be useful to people outside the Amiga community as well.