Don't take this to the bank, but it seems that the slats running parallel with the frames is favored.
Lot's of confidence in my statement, eh?
As for the robbing... I think this is attributed to using open bottoms in conjunction with the slatted bottom. In other words, the colony enters and exits through the area of the entire bottom (through the slats). There is no bottom board below the slats making the hive open at the bottom except where the slats are. I would imagine this gives a formidable force for robbers to reckon with...lots more bees staring the robbers in the face than would be at a normal front entrance. Mr. Bray has mentioned to me that he believes robbers from *non-slat bottomed* colonies appear confused with a slatted open bottomed hive...other colonies that were house in slatted open bottomed houses seemed adept at robbing from open bottomed colonies.
If you can get Mr. Bray to respond you'll get lots of good, experienced information from him!