If you intend to really use your FF then you want the biggest screen you can afford. I don't know what features come on the Lowrance but on my Humminbird 999 I can display three different screens at once. Any combination of SI, DI, 2D sonar and Maps. Trying to display two or three different views on a 5" screen just isn't practical because you can't see the detail.
Check around to see if one of you fishing buddies has a large screen unit and ask them or go by one of the big outfitters (Basspro/Cabelas/Gander Mountain) and spend some time comparing working display models to get an appreciation for the difference screen size makes.