Hello again Amber, you ask great questions! To start, the XI50B is the blade-based version of the DataPower XI50 rack-mount appliance. So XI50B fits into a IBM BladeCenter slot. See my article at
www.ibm.com/developerworks/websphere/tec... about the XI50B.
The XB62 is the latest generation offering of the prior XB60 product, which is for all intents and purposes the XI50 with some B2B capability added on (ebXML, AS1, AS2, AS3, transaction viewer, partner profile management, on-board transaction database, etc).
So XB62 is almost a superset of the XI52 as the XB60 is of the XI50 (the older generation of the two). I say 'almost' because there were a few obsolete object types pulled out when they built the XB60 and XB62.
So if you need that type of B2B capability now, or might in the future, I'd go with the XB62. Ask your IBM client team to set you up for an in-house POC with our DataPower tech sales team!
Also, there are some great redbooks on the IBM redbooks site on the B2B appliance.