There may be arguments in favour of either solution, depending on the types of data involved and programmer preference, but if you are to embrace the full Ensemble "model" then I think the second option is far better.

By putting the non-HL7 into a message sent to a business process,  it gets stored and becomes visible in Ensemble in it's raw form (or as close as you can make it) on the  message queue into that Process. This makes support much easier as you can see before and after messages in the Ensemble GUI. Also, a Business Service should do a minimum of work so that messages are input as fast as possible.

Using DTL is also the cleanest option, since it is meant to transform messages, but I admit that sometimes this is more effort than it is worth. I have had to deal with complex xml documents, and ended up writing methods in my custom message class to make extraction easier to understand in the DTL.

Mike