Magnaflow Cat: 23909 vs 49150

Any reason to order the $600 Euro-4 version over the $300 Federal version? I understand that the Euro-4/California version has lower emissions, but I only care about passing the OBD2 test and durability. I am just looking to get a couple more years from my stock '04 commuter RX8 with a currently dead (or dying) cat.

TIA