Go for square/rectangular with the heating element/fan unit in the back. It gives more even drying. The problem with the round ones is that the heating element is on the bottom, so you end up needing to rotate the trays, or you'll end up with the stuff on the bottom getting overdone, and the stuff on the top not getting thoroughly dehydrated. Plus, there have been several instances of the round plastic ones MELTING.
I have had both types, and now just use the square ones, but I do NOT have an Excalibur - I don't like them because they are made of plastic, which can melt. The model I have is from Sportsman's Guide, and it is made of METAL (so it WON'T melt). 10 wire trays, approximately 13" by 14", with temperature that can be adjusted anywhere from around 90-95 degrees for dehydrating herbs, clear up to 165, for dehydrating meats, and everything in between. If I need to dehydrate something that is likely to fall through the wire as it dehydrates, or is wet, I either use parchment paper on the trays, or some flexible nylon cutting board that I have.