I really agree with this post. I'd also add that since it sounds like you spend a lot of time riding in group rides I'd say go with the Tarmac. If you're drafting off someone else for a large part of a ride, what's the point of spending money on an aero frame when you're catching a draft off another rider in front of you. And unless your name is Mark Cavendish I doubt the aero difference between the two frames will result in your winning or losing a town sign sprint.