Hooded sleeveless vest - would you choose warmth over flexib

You are sufing cold water (42-45 degrees) in a new 5/4 high end wetsuit and need a hood so you are buying a hooded sleeveless vest.
You can
A. buy a 0.5mm neoprene vest which adds bulk and feels bulky but is much warmer.
B. Or a very thin, very stretchy and flexible polypro vest which also has a hood, but adds only a tiny bit warmth to the core.
Whats it gonna be?
You can
A. buy a 0.5mm neoprene vest which adds bulk and feels bulky but is much warmer.
B. Or a very thin, very stretchy and flexible polypro vest which also has a hood, but adds only a tiny bit warmth to the core.
Whats it gonna be?