I’d say stainless coils are better for 1-2 taps but more than that the cold plate is the way to go, you just can’t fit that many coils in a box. The coils are lighter and they give more contact with the ice. The cold plates are just stainless tubing in an aluminum plate so you don’t have to worry about your beer touching aluminum.
It is optimal to cover the coils completely with ice, especially if you are serving constantly at a busy festival.
The cold plates I have are essentially (IIRC) 18’ of 3/8" tubing per line. So you can calculate the PSI required from the tubing type/size. Of course it varies a little every time, on my boxes I usually start at 25psi and if it isn’t pouring properly go up till it does.
Personally I would check ebay and craigslist for a while before buying new. I picked up a whole 2 port coldplate setup for $75 on craigslist once and then later found a six pass cold plate on ebay for about the same price.
Another thing with cold plates is that, yes you need to have them covered with ice, but they also should be mounted about an inch above the bottom of your cooler and the water should be able to drain out as the ice melts.
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.
I have a 2 pass cold plate that works pretty well. It is 25 ft of 1/4 in tubing. I have found a sweet spot at around 18psi. Also, it works MUCH better if the beer is at least cool in the keg. It is hard for me to pour a warm keg of beer through the plate without some foaming.
I have to disagree a little bit about the water/ice ratio. I think it works best if the plate is surrounding by a little bit of water, with lots of ice all around. If you drain out the water, then you are losing some contact area with just the chunks of ice in there. It probably doesn’t make much difference in the end.
By the way, instead of buying a newly assembled jockey box, you can easily convert a cooler into one of your own for much cheaper if you can find a used cold plate on ebay. If you can make a kegerator, then you can make a jockey box.
Personally, I think a cold plate gives better results. More surface area, more mass, and aluminum rocks as a conductor. I have 10 ft of 3/16" lines between the kegs and the plate, and 10 ft between the plate and faucets. That’s ~15 psi of pressure drop, plus ~5 psi for the plate itself (assuming 18 ft of 1/4" tubing). So I can serve at 25-30 psi, which keeps the keg carbonated in case it doesn’t kick.