Before buying a Platinium be sure that you're unit can read the Platinium. The Elite series can't.
Compatibility list: http://www.navionics.com/sites/www.navionics.com/files/documents/Americas_CompatibilityGuide_2.pdf
For the Platinium users don't forget to activiate the Fish N Chip function to benefit the Sonarcharts!
If you purchase a new Navionics don't forget to register you're card within 60 days of purchase on the Navionics webstore to get a full year of Freshest Data.