Work the question backwards........
A Chart plotter is designed to make navigation at sea easier, so start with the Charts.
Garmin uses Bluecharts original, G2, or G2 Vision.
Raymarine uses Navionics
Furuno uses Navionics or C-Map
Where are you going to cruise? What type of vessel? (e.g. a sailboat will probably have Raymarine instruments inboard which may suggest going with their Plotter).
A "Form follows function" approach may prevent you being seduced by some of the current bells and whistles that may be of little practical value.
Getting a relevant copy of "Practical Sailor" would be a great place to start.
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