BB Slim (Fitz) is cut too tight for my lats and delts and then is huge in the waist. My shoulders extend way past the should seams, and my lats get dug in to by the armhole.
The SM Slim is a better fit for me up top, and then slimmer through the body. I should say that I'm not one of those "skin tight shirt" guys. The SM slim provides exactly what I would describe as a normal, but slim, fit.
Too each their own. I love the Fitz fit (after some waist suppression) but I also have narrow shoulders.