Actually it depends on how the CPU will be used... The FX-4100 pricing is very close or lower on the FX-4100 so it's the OP's call.
According to AMD, they have no issue with mobo makers approving their mobos for use with FX series CPUs though AMD will only recogmize those mobos using the proper 900 series (AM3+), mobo chipsets designed for the FX series.
You should confirm with ASRock if an FX-4100 CPU will actually run on their AM3 mobo and if a BIOS update is required - which is probably mandatory as it has been even for AM3+ mobos sold before the FX series was released.