Actually neither - it depends on your shape and style. The buoyant neoprene you choose to insulate yourself can make a real difference to your swim times.
Where is the buoyancy of the suit situated relative to your unique water position, especially when fatigued - do you really know?
A well looked after suit can last a long time so make the right choice - it's a matter of sink or swim. What can you share that will guide others to be less brick more slick?