EXPENDING FOAM: Open & close cell foam?

I have two options: buying close cell, fire resistant, R-7 per inch, 3 lb density foam for $12 per cubic ft. OR buying open cell, fire resistant, R-3.7 per inch, 0.75 density foam for $6.80 per cubic ft.

SO, I would like your educated opinion on what should I buy. Thank you.

P.S. I’m using foam with combination with fiberglass, just what to seal all small gaps to air proof. I do understand difference between open and closed cell

Closed cell foam is air tight. Imagine thousands of little balls (each sealed with air inside) compressed together. Open cell foam is more porous. More like a sponge. I personally prefer closed cell foam.