If you're curious to see the differences between NieR Gestalt for Xbox 360 and NieR Replicant for PS3, then you'll need to find a young rich Japanese widow to marry and become a citizen in the land of the Rising Sun. Only Gestalt is releasing outside of Japan, with Replicant being a Japan-only deal. At least, that's the implication.
While not specifically stated, Japanese-to-English blog Andriasang is reporting that a Famitsu interview with executive producer Yosuke Saito "heavily implied" the international status of NieR. Considering it's only ever been called NieR in the West, with no mention of a Gestalt or Replicant, it's an implication that is quite likely true.
According to the interview, NieR Replicant is designed to appeal specifically to the Japanese market, with Gestalt being designed with a more Western slant. This explains why the game's titular character is younger and more effeminate in Replicant, and why the main character is his sister and not his daughter. Japan loves a girly virgin.
In other news, NieR will be a single-player affair, with no co-op planned. I approve of this, because I'm sick of shoehorned co-op in my action games. Also, DLC is being considered.