Definitely go with the S5, but pay attention to the frame level you buy, as Cervelo makes three different grades (Base, Team, and VWD) of the S5 frame. The frame which is standard with the Rival group is the base grade obviously, to achieve the $3800 retail price.

IMO, you would want at least the second level "Team" version if you could swing the cost, but it comes standard with Ultegra. My dealer told me many S5 buyers are getting the Team/Ultegra bike and then removing the Ultegra, then eBay/Craigslisting all of the components and then gearing up the bike with Rival or Red or whatever they want.