At 2100' or less I prefer turn islands. Without I feel rushed, especially if you do a traditional gate. Its also nice to have the time to adjust your shorts, thnk about the pass, etc. the maintenance can be a pain. Where I ski I'm in charge of mowing the islands and spraying weeds. Doesn't take that long, but when you're pressed for time it can get tough. As for depth, go as deep as you can afford. The lake I ski at, also in MT, is 5' deep and is loaded with weeds. Especially if it doesnt get used much, the weeds will grow that much faster.