Good Morning, I'm working on pulling the trigger on a HB 798 unit and I'm wondering whether or not I should get a card for this unit? I already have the Navionics app on my Android phone so I'm curious if there is very much difference in that app compared to the card. Also, if I use this card on my unit and say we go out on my buddies boat, can I use that card on his 898 or will it only work in the original unit it was downloaded too? I was just curious if they were worth the money and what the limitations were on them. Thanks for the help.