Is it better to use torches or slabs to cover the roof with to stop mobs from spawning?, I know both stop spawns but do slabs slow spawns down because the game checks it constantly with it being dark enough or does it make no difference?
In your scenario (slab on full block versus torch on full block), the torch is better. The torch, I'm pretty sure, is classified as transparent, which means it will not impact spawn rates. The full block and slab are both classified as non-transparent, so they will impact spawn rates; basically the slab on full block is twice as slow as the torch on full block.
That said, all of the above is happening on a scale that you will never actually notice without some serious instrumentation. It's far more important that you don't put anything but transparent blocks above the slab or torch, and that you build the grinder as low as possible. These two things will significantly speed up the grinder, but obviously in a normal world the trade-off is a lot more work in the form of digging and spawnproofing. To put it in perspective, a 16x16x16 grinder placed at the bottom of the world is roughly 3 times as fast as that same grinder placed at sea level and more than 5 times as fast as that same grinder placed at world height.