The 2.3 all the way.
I have said it before and I will again: The 2.3 is an almost perfect all weather/winter training bike.
Pros:
-Inexpensive: so, I am not going to cry if I crash it or beat on it.
-Stiff: Excellent well tested Aluminum frame and a Shimano 105 crank, (which is one of the stiffest you can get). Both the 3.1 and the 4.5 have lesser cranks IMHO. Although I wish it was spec'd with a standard double instead of a compact.
-Durable: Again Aluminum frame, and easy to replace and find parts. Wheels are just right for tough and crappy training rides, and you could race on them too.
-Easy to upgrade, (not that you have too): you can easily intermix and incrementally upgrade parts from Ultegra or Dura-Ace.
-Training/Winter Bike: Hell, it has fender mounts!!!!!! Can't tell you how cool that is for a training/winter bike.
Cons:
-Crankset. While the compact/11-28 cassette gives you essentially the full range of a standard double with a 12-25, I, particularly, don't like compacts and the wide spacing they require. Especially for a winter bike, I would rather have a tighter cassette. I suppose you could just do a cassette change (12-23 or so) and loose the big gears, but why not just spec a standard double and have it all.
The 2.3 is ready to ride out of the box, after pedals. Perhaps a new saddle (which you would probably do anyway), ask LTD/LBS to replace the compact with a standard double, and a better cassette, and that bike has all it needs. Plus, I have never heard a bad word about the 2.3 from anyone who has some miles on one.
Did I mention it has fender mounts?!!!
HTH
zac
EDIT: Maybe one of the owners can pipe in here, but the 2.3 with 1700gram training wheels and a reasonably light saddle can't weigh much more than ~17lbs. if that matters to you. Out of the box it is probably around 18lbs., which is excellent by the way.