Many people (including myself) use June 2007 as their diagnostic b/c it's free and somewhat modern (2007). I really don't think it matters which test you take as your diagnostic.
I tend to go against the TLS grain in thinking that the older tests are not that much different form the newer ones. Especially with the trends in the logic games section, even an old test would be fine to use as a diagnostic. Old tests are also fine to train with, in my opinion. Although I do find newer RCs to be harder, but once again not to the extent that most people do.
I find the older tests have a different way of wording questions which makes it seem harder to some people. Unless I'm just crazy and it's completely the same...possibly...maybe??