Playstation Plus games, and physical copies?

So with Playstation Plus they offer their 'Instant Game Collection' where you can download games for free, and if you downloaded them atleast once, you can have them even when they are out of rotation so essentially you have them forever. Among these games currently in rotation are Uncharted: Golden Abyss,... show more So with Playstation Plus they offer their 'Instant Game Collection' where you can download games for free, and if you downloaded them atleast once, you can have them even when they are out of rotation so essentially you have them forever. Among these games currently in rotation are Uncharted: Golden Abyss, and Gravity Rush; I previously bought the actually game cards, but now that I have Plus, I should be able to keep them. On to my question...Should I just sell my physical copies of the two games, and use the money to get a bigger memory card, or am I misunderstanding how Plus works? Thank you so much for taking the time to read this, and I appreciate any help.
Update: Well honestly with only being $18 every three months, I plan on being a Plus member for quite some time. It just seems like too good of a deal; can't see why I wouldn't want to stay one at that price.
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