Welcome to the forum irf3.
There is plenty of information already posted here about the Giant TCR Advanced. Since these bikes are basically unchanged from their introduction in 2009, some of it is a bit older, but still as accurate as ever.
Good threads to start with are:viewtopic.php?f=3&t=47893
All of these were found searching the road forum for any threads with "giant advanced" in the title.
Personally I've been on a Giant TCR Advanced SL since 2009 and can't imagine a better bike for my style of riding. It has a wonderfully stiff BB, fantastically accurate front end and the ISP gives a ride quality that I noticed as being better then my previous bike (a 2007 Giant TCR Advanced).
Given your price range, I'd personally be tempted to see if it was possible to find an older 2009 or 2010 TCR Advanced SL - though I certainly have heard only good things about the non-SL Advanced, and doubt that you'd be disappointed by it's ride either, I have heard that the aero-post (which you'd be stuck with), reduced the compliance of the frame.
Basically, if you could ride these two bikes back to back you'd notice a difference in ride quality and probably a few in performance too. When you then consider that an older SL would cost the same as a new non-SL - it makes a strong argument for the former.
Sorry that I can't provide anything on the Jamis, but I don't have any experience with the brand. That said, I'm sure a search of 'jamis xenith' will give you a good place to start.
Good luck and please keep us posted on what you end up with.