I have a Gran Fondo on order. I read around a bit as I was undecided between the Sportive and the Gran Fondo, the former supposedly having a carbon steerer which accounts for the different quoted weights.
I've gone for the 56cm frame (XL?) as a couple of guys on bike radar have the Gran Fondo with that frame size (sat on the Ribble test rig to measure up) and who are a similar height (5' 11"). I would have gone for the 55cm (L) Sportive.
http://www.bikeradar.com/forum/viewtopic.php?p=15956981
I went for the Special Edition as it has been cheaper for me to shop around for flashier finishing kit than to pay Ribble's upgrade prices.
I've gone for an Ultegra set up and then bought separately an Arione CX (Planet-x), Bontrager XXX Lite seatpost (All Terrain cycles) and Pro-lite Bracciano wheelset (Chain Reaction).
I've also bought Planet-X's carbon/titanium QRs and carbon bottle cage.