I've got a Roubaix (expert) and a Tarmac (comp) and I'm equally slow on both (I get dropped on the hills every tues. night). Of course, neither bike is an S-works, so maybe I just need to upgrade so I can be fast. But seriously, the main difference I feel between the 2 bikes is the handling (Roubaix is less twitchy) and the ride. The Roubaix is smoother on chip seal, etc., but I don't think the Tarmac is harsh or anything. I don't have enough power to feel any difference in power transfer/frame flex, but the Tarmac has the edge in theory. There are also some significant differences in frame geometry, including a much shorter head tube on the Tarmac. Make sure this will work with the saddle/bar drop you are looking for.

If it's not a lot of $ for you, I say buy the Tarmac since you already have a Roubaix. It won't make you immediately faster, but it might shame you into working a little harder (who wants to be the slow guy on an s-works afterall?). If you want to get faster/stronger and don't already have a powermeter for training, then I'd say buy a powertap wheel before buying another bike. Or help the economy and buy both...