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.
I noticed Jason McMullan is taking interest in developing for the Natami, which is great news!
With all this talk about AAA and SAGA and Natami being in alpha stage I was wondering how fast the FPGA chip is running at (over 145 mhz I hope) ... I would love to see a 1080p resolution for Natami.