Whichever one you get make sure you get the 2008 updated version which should be available I believe around June. When they came out with Red last year they made some mechanical improvements which filtered down to the Rival and Force groups for 2008. That said I would go for the Rival group. Lighter than Ultegra and the 2008 version has carbon brake levers.

I have been riding the Rival group for about 9 months and I like it. I might just update the brifters (I hate that word) and the whole setup I believe should work like the 2008 group mechanically. Though I am not 100% sure of this.