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.
Take a look at my answer to this question:
The difference between the RAM modules you have is UDIMM (unbuffered) versus RDIMM memory; unregistered versus registered. They cannot be mixed in the same server. So you are correct, the memory guide states not to mix the two. http://h18000.www1.hp.com/products/quickspecs/13344_na/13344_na.HTML#Memory
Do not mix Unbuffered memory (UDIMMs) with Registered memory (RDIMMs).