Either Lowrance model you mentioned would be goodl. I have the 67 mounted in the front of my boat and an X15 by the steering wheel. If you were choosing one I would go with the x96, you will like the bigger screen if it is your only unit. The little color unit is nice but I hate the small screens. With the bigger screen you can run it split, on half you can have a normal view and on the other half you can have a 4x view of the bottom, that would be very handy for walleye. You can run the 67 the same way but with the small screen it would be hard to read.
The only advantage you are going to gain with the color unit is the hardness of the bottom and it is easier to see.