I recently purchased the 2019 S-Works Tarmac. Before doing so, I test rode the regular Tarmac for a day on a 90mi/6kft ride - long hill climbs, high speed descents, rollers, and flats on varied pavement to really test it out.
Both the regular Tarmac and the S-Works version have the same geometry. But that’s where the similarity ends. The carbon fiber materials and carbon layup are different and that leads to some very important performance differences. The regular Tarmac is a great bike that most people would enjoy - I certainly did! However, the S-Works Tarmac is in a different class - it is a racing machine. In the “old days”, these differences would just translate into a stiffer frame. That’s not true anymore. Engineering is so advanced, that when you do sprint accelerations, it has a real “snap” to it. Power transfer is amazing.
If you want to see a few #s from my first 4 rides on the S-Works, see my post in the Forum thread below that includes a snapshot of my WK04 PDC comparing the past 2 weeks vs 2019. Unfortunately, there wasn’t a power meter on the regular Tarmac for comparison. The comparison in the #s is to my Specialized Roubaix that I have been riding (a great endurance bike).
I think the bottom line for your decision is whether the kind of riding you do/plan to do is one that could utilize the S-Works and whether you can/want to spend the money on it. In either case, you will have a great bike.