The ability to move your zoom up and down the water column would help on kokes, rainbows too. Actually any time the fish are suspended. I use and appreciate that function on the Marcum.
As far as interference, the people using Vex's around the Marcum are affected more than the guy with the Marcum
.* It's got a lot of power and decent interference rejection.
Any of the models mentioned so far would raise your awareness of fish behavior and get you catching more fish. They all have their pros and cons.
The best advice I can give is to be sure you get one that you'll be happy with. Don't save a small amount of cash to get a unit that you'll want to replace after you see features on another unit. You'll just end up wanting to trade up, and that costs more in the long run.