How does your Tarmac fit? If your fit is compromised, the biggest difference will be a custom fitted bike versus an off the rack bike, not frame material. They are both good. They can both be designed for a particular purpose and end goal. If your priorities are being noticed on a flashy, one of a kind bike, stick to centuries.

The SL4 is a very nice carbon frame, but it doesn't hold a candle to a top carbon frame. The differences are significant. I went from a Felt F5 (1005 grams) , which is similar to your Tarmac, to a Felt FC (938 grams) last year. Same manufacturer, same market niche, similar geometry. It's much lighter, yet much stiffer, handles quicker, and rides smoother. No comparison.