Which one has better resolution ?

The one with the better resolution should be placed where you spend most of the time fishing and not where you spend the least ammount of time which is driving the boat.

The 640c isn't a big wide screen unit, it has a 5" screen.

The transducer for the 320c may not be interchangeable with the 640c.

I would use the 640c up front where you fish. You should be considering a good sonar/GPS unit for the consul.

To get the most from your unit, learn to operate the unit manually in lieu of on auto. Remember the area of the signal return is only 1/3 of the depth of water you are metering, directly below the tranducer location.

I would also set both units up on RAM ball mounts for easy viewing, removal and interchanging units, if the TD's are compatiable..

WRB

Good advice Raul, maybe some day he will learn to locate and fish for deep fish and the higher pixel count will show the fish and his lures better.