Hi, the Xbees are XB24-BCIT-004, which are Series 2, chip antenna, radio data rate 250Kbps. The path length is about 2'6" I'd suspected that it was something with packet collision/overlap - I'll have a poke and see if there's anything to change with RTT. It's not critical, I got the Xbees for a greenhouse temperature sensor and just "repurposed" them to remind myself how to do serial comms. I haven't used the XBIB; just cheapie USB boards.
I've got both the RasPi and a BeagleBone Black, though I've used the Pi more. (I've got 2 of model a sitting waiting for me to solder up the TNCpi kits, which is what prompted me to join this group.) The RasPi is (slightly) cheaper and a lot better at multimedia - XBMC and the like. The BBblack seems to have more resilient, and just more, GPIO. The BBblack has a much nicer hard real time system, the Programmable Reatime Units (PRU). The LinuxCNC guys love it. The RasPi seems to have a much larger user base, and lots more accessories. For example, the Cooking Hacks "Raspberry Pi to Arduino shields connection bridge" which has Xbee socket connecting straight to the RasPi; or the TNCpi. (I'd love a TNCbeagle!) The BBblack has support for "USB gadget" api, so you can program it to act as, for example, a USB keyboard, mouse or network dongle. For talking to an Xbee I'd go for a RasPi, but pretty much for the feeling that the BBblack was too good, and the RasPi, model b, has 2 USB sockets. Bill (M1BKF)