A few [2 French, 2 Dutch & a Pole (or 2) at the last count] but there must be (many) others who never post.
...and a German...
As for more specific questions, should I ask in each model's subforum?
...yes, you should...
...it depends on what you are looking for exactly, but...
...the european export STS was only available from MY 2005 to 2009...it's no problem to find a 2005 and 2006 (and many do have a late registration) and some 2007, but the facelift-2008 and particular 2009 are very rare/seldom here...comparatively...(MY 2010 and 2011 n/a...only grey marketed...if any...)
...the 2nd Gen CTS as an european export was available from 2008 up...they are still "very" frequent...
(and the euro export specs MY 2014 model change 3rd Gen CTS is only available as a 4 cylinder 2.0L Turbo)
I'm thinking about getting a used STS or 2nd gen CTS (both V6), what are the main differences? Other than the CTS has better fuel consumption.
...in a direct comparision...the STS is a little more "out-dated"...
For euro export specs cars:
...if you are looking for a V6, the 2005 - 2007 STS got the 257 HP (DIN) 3.6L LY7 engine, 2008 and 2009 got the 3.6L Di LLT with 311 HP (DIN)...
...the 2nd Gen CTS startet with MY 2008...and had more types of a V6 available...
...for the 3.6L on the export specs CTS...there are a few early 2nd Gen CTS, that still got the "weaker" LY7, but the most got the 3.6L Di LLT...(also available a 2.8 and a 3.0 V6)
...and 2012 the V6 engine was changed to the 3.6 LFX with a 322 (DIN)-HP rating..