The BM54 has a 'Region' setting, just like the BM53, but it's locked to Europe, so it won't work in the US.

The BM54 does not support Sirius, it does not recognise the iBus device ID.

Also, but I'm not 100% sure about this, the BM54 includes a 4x15W amplifier but the BM53 does not as all US equiped cars have some form of external amplification, the Euro E46 does not.

Unless you have a VERY good reason, strick with the BM54 in Europe and BM53 in the US, probably the BM53 in the rest of the world too.