I have built both flavors... I have a double tap ss coil version, and a 5 tap plate chiller version.
As others have stated, you can only get about two coils in one cooler. The ss coil can hold more volume of beer. This is advantageous if you are serving a lot of beer frequently. It works best if the ice and water remain in contact with the coils.
The plate chiller would be my choice only if you wanted more than two taps on your box. These work best with constant ice contact. Keep the drain open and occasionally knock the ice around the plate to get it to fall down on the plate.
As far as keg pressure goes, it depends on the temp of your kegs. Remember, to keep gas in solution, you must increase pressure as the temperature is increased. This tends to go against your instinct to keep lowering the pressure because of foamy pours. Generally, my experience, with warm kegs that I need to serve at 22-27 PSI on my setups to keep foaming down.