Additionally, if you want a particularly cool look, pretty much every single Xycraft block will prevent mob spawns regardless of light level. From what I've heard, if the block does the weird "glowing" animation it will prevent mob spawns on it. (But it does NOT emit any light unless you're using the actual light/lamp blocks.)You mean in the nether? Or elsewhere? Anyway to prevent mob spawning, yes, vanilla slabs work - but more efficient (material-wise) are Redpower2 Microblock 'Covers' which have the same effect.
Obviously that's way more material intensive. It depends on if this is going to be a place that you want to look nice (which could still be achieved with slabs of course) or if you're going for a quick and dirty solution (which torches would be even better for IMO).