I think most would agree that "thickness" means the same thing as "viscosity". In other words, a more "viscous oil" is considered a "thicker oil". Now both type of values you list above (HTHS, cSt @100°C) are a measure of viscosity, but under different conditions. The major difference between the two is, HTHS is measured at 150°C, and, it's measured when the oil is flowing through very small dimensions so there is a lot of "shearing" taking place, thus the name "High Temperature High Shear".
Another way to look at it is that "cSt @100°C" is a measure of how oil flows through the plumping in your engine, and "HTHS" is a measure of how the oil flows in your engine bearings and highly loaded places like the valvetrain and piston/rings-cylinder interfaces.
Ideally you want a "cSt @100°C" that matches the flow requirements of your engine with a high HTHS for wear protection. In the case you mention above, the Castrol RS 0W40 is the winner in my book since it flows better, yet offers better protection under high stress conditions. One would also want to look at the cold flow viscosities to round out the picture, especially if you will be doing a substantial number of short trips in cold weather. And finally the additive package will influence the total package substantially.
