While the smaller screens are cheaper, they can make it a bit tricky when bouncing about in a smaller boat. Personally, I wouldnt worry too much about the charts themselves, its the ease of using the plotter that makes a difference. Raymarine is very good, but expensive, and Garmin is good too.. with Garmin you should get some UK charts pre-loaded, which makes them pretty good value. Few years since I used Lowrance, but needed to manual every time I wanted to do something! Might be better software now, of course.
Most of them you sort of hard wire, then use with a mount/stand; then you just unscrew the cable connectors on the back of the plotter and take it home. Takes 20 seconds.
Personally, I d take a good look at the Garmin 4-5 inch screens.
(Oh, just check any plotter you get does include the gps antenna, either built in , or separate, or you will have another bill!)