At the end of the day, I don’t think it matters too much which direction the closed loop flows. It comes down to operational simplicity, in my opinion.
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You want to evenly fill the column slowly. It is kinda hard to take a 6×48 material column and fill it evenly without voids with a 1/2" line . You need to fill the headspace of the column first so it forms a pool above the material then slowing move that down the column . That’s why on a 6x48 we need to run so much more gas to get a good efficient extraction or soak forever . I remember back in the day when we typically ran 3" or 4" at a 3:1 or 4:1 ratio of solvent to biomass it would fully strip all the goodies . Its because it is way easier to fill a 3 " column without any voids compared to a 6" . A shower head is not going to help on a 6" column too much surface area and not enough liquid coming in to achieve this. This is why everyone runs 6:1 or 10:1 on 6x48 columns . Its really just about evenly transfering the liquid solvent over the material without any voids which is hard to achieve with 1/2" lines without taking for ever on a 6x48 .
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This though 
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I run wide screen mesh gaskets on top just for that reason. Also ive ran top and bottom feed with a shower head and it never clogged in over a yr.
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