Seven inch vs five inch display

I like the 5" units because the mounts they come with can be rotated for viewing from the bow, the width of the 7" screen doesn't increase the distance you can see in front of you, cost less, and can even be flush mounted in a small console.

But I'm always fishing where I can see the bottom or the fish and don't use the fishfinder features, so I can't speak to the split screen benefits.

 

Ask you wife what size she like better....

 

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.