Personally I enjoy my Sram red... But I see the benefit of Ultegra Di2 as a future upgrade for my girlfriends bike for the sole reason of the auto trim on the FD.. she can't figure out how to trim as it stands...
http://www.merlincycles.com/bike-sho...pset-2013.html
http://www.merlincycles.com/bike-sho...-groupset.html
and the price differences between the two are nil...
If you do go red, make sure you go YAW FD with the new 2013 Exogram crank as you will find the shift far superior than 2012/2011 with the Ti front cage.
Also the XG-1090 cassette is amazing quite compared the the old PG-1090's.
I started my RED upgrade as a 2011/2012 complete and as I came across good deals on new 2013 stuff I made the switch and it was a night and day difference. only 2011/2012 stuff I have left is the RD and shifters.