XBP24BZ7UIT-004 - is an XBee-PRO ZB S2B module, developed as a Router in AT mode. Refer the following, http://www.digi.com/products/model.jsp?mid=3588.
XBP24-Z7UIT-006 - is an XBee-PRO ZB S2 module, developed as End Device in AT mode. Hope this helps.
The XBee-PRO ZB S2B is a newer hardware, and I think the S2 is older thus you won’t be able to get them after some time.
I also think the XBee-PRO ZB S2B has better software control of TX power at wider temperatures, which is more important in Europe with the many rules limiting power.
Both should run either router or end-device firmware, so i don’t think there’s a big difference there.