I was told that there are 3 different kinds (densities) of Grodan and they are labeled Delta, Classic, and one other inbetween the two. Apparently the Classic which is usually the slabs drains quicker where as the Delta which are usually the blocks hold more water... this would make sense if you are starting in a block then setting that block on a slab, you would want the slab to drain quicker. This is what I was told and was wondering if anyone could confirm this info?
As far as I know, the only difference between the Grodan labeled products are that in some slabs, the fibers run horizontally where they are vertical in all other Grodan rockwool products.
I find that Grodan 4x4 cubes work great. Tried the Pargro, but it doesn't hold as much water as the Grodan, that's why it is cheaper. Note that rockwool MUST be submerged in water for 24 hrs before using. If I skip or shorten this step, then I will be pH downing the reservoir for several days and - far worse - the blocks will not wick up nutrient solution to their full potential. Properly prepared, these cubes will wick up water to their tops with only a 2.5" flood depth on an ebb/flow table. They are flooded twice during the 12hr light cycle for 20 minutes each time.
In order to more easily move the plants around, I put the rockwool blocks in square pots that have about 3/8" clearance around the block. The plastic wrapper is removed from the block and a black/white plastic film block cover goes on the top. The effect of this is that the roots shooting out the side of the block are somewhat air-pruned. I cannot say for certain that it improves yield, but the square pots definitely add stability and ease of moving with top-heavy plants. Moving them around for maximum canopy exploitation definitely improves yield.
