All 4 SI units are great but the bigger the screen the easier it is to see the small details in the data. So the 1198c will be easier to see the small returns like fish versus the larger screen of the 1198c.
I would get the biggest you can afford.
I currently run the 998c on bow and 1198c at the console. And ran the 898c on the bow last year. All three are great units.