Hi The advantage of the Spektrum DX6I is if you have a Tamiya pershing (which I have )it has a very silly startup and operating ,You have to go full reverse to get it to move,but also to make it fire it is full stick movement plus trim to fire any of the guns,(with the Spektrum you can set this to happen without using the trims or even use channels 5 and 6 to make it fire)
Strato50 is correct you cant add more channels to the radio,this is why you need to sort out how many channels you will need for the boats,2 channels to control the speed and direction,leaves 2 channels with full, control turn turrets, elevate guns and 2 channels to turn on lights and radar.if this is all you need it is one of the best on the market.

regards pete