I am in the market for a new SWorks frame. I have been riding a Chinese import that is a 55 cm frame. Seemed small at first but it handles ok. I am six foot, between a 31-32 inch inseam.
In the past, I had the first all Carbon Sworks Tarmac in a size 56. It fit fine, with a 110 stem. but it felt great when I had it fit. Now, I am seeing lots of US Pro's and European Pro's riding what seems to be small frame sizes. A size 54 Tarmac seems about right to me standing over it and next to it. The 56.5 which all the local sales guys say should fit me, but it seems big. I like a 120 MM stem, and have very little set back on my saddle right now. I know I can make the 56 fit with a shorter stem. They come standard with a 100 MM stem, which I think is ridiculous. The 54 could fit with a 120 stem and a semi normal set back on the saddle.
I really want to hear from the SWorks owners who are about six foot tall and what size they are riding. Any help I would appreciate!

I'm 6'1" and ride a 58 with 110 stem (S-Works SL3 Tarmac). My fitter wanted to move to a 120 stem but I'm getting older and the 110 feels just fine. But at 35" inseam, my legs are significantly longer than yours. Height alone doesn't define your needs, as you are aware. But at your height of 6', I would think the 56 is just about right. If you're buying an S-Works frame the shop should give you a BG fit and that should provide your answers.

What's your saddle height?

I'm 5'11" and ride a 54cm sl4 with a 760mm saddle height. I've got a -6 120mm stem on, 30mm of spacers underneath and about a 100mm drop to top of bars. I'm using a 25mm laback post clamped center rails on a romin.

Should give you a pretty good idea of how it fits.

I am 5'10" on a good day and ride a 54cm with a 130mm stem and just the 15mm conical spacer underneath.

What you're describing sounds upright/small to me. 6ft wld normally be on a 56cm, even allowing for running a frame slightly on the "small" side.