They sure look "Legoable". On the other hand, the body probably only accounts for a very small part of the cost of the light so I don't know how much of a discount you could expect to see from buying just the head.
You may want to ask the reviewers who have both. I only have the TN31 on the way, as I already ordered the Xtar S1. I couldn't justify both, well at least not yet :devil: