Lowrance Fishfinder question

[Big Santa]

I am considering an X15 or X91. By far, most of my fishing is trolling Lake Michigan for salmon and trout. I do some trolling for walleye on Lake Winn. and Little Bay de Noc.

When I am trolling Lake Michigan, I am mainly fishing suspended fish in 200 feet of water or less. To me, the important factors in the fishfinder are:

1) Ability to mark bait when running
2) Ability to mark fish when running
3) Ability to mark thermocline
4) Surface temp graph is helpfull
5) Ability to mark my balls (downrigger, that is)
6) Obviously, mark bait and fish when trolling.

I used a 350A on a previous boat and was very happy with the performance of the unit.

I have a seperate Chartplotter and will not use the GPS funcionality of the X15 if I go that route.

For those who have used both units (X85 is almost identical to the X91 as I understand it), am I giving up much by going with the X91 over the X15 (considering my type of fishing)? Having previously used a 350A, will I notice any "reduced performance" in the X91?

Bottom line (no pun intended), I love the screen of the X15, but will it help me catch more fish than the X91?

Thanks for the input.

Big Santa.