I don't quite understand the differences between the two besides the obvious price differences. What does each offer over the other?
Dell has a nice forum article about the differences. RDIMM is REGISTERED memory and UDIMM is UNREGISTERED memory. This means a few things, UDIMM is limited to two DIMMs per memory channel and UDIMMs offer slightly better memory bandwidth for one DIMM per channel. However when using two or three DIMMS per channel you will get better memory bandwidth with RDIMMs.