They do the same thing the same way with the same transducer.
The smaller display will have a higher pixel density and sharper looking images up close. The 1199 will display stuff in a larger size with the same unit settings.
If you end up with the two sizes, put the 1199 at the bow. BIGGER is better from a distance.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1