12" Fi BTL vs 13.5" JL w6

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I am currently running two 12" JL w3s in my car and I am planning on upgrading. I want to be able to hit lower frequencies and have louder bass. I was looking at buying 2 13.5" JL w6s when I discovered Fi's subwoofers. I have watched a few videos and searched online and I was hoping that someone here could give me some advice. I was going to get a ported box and whatever amps and wiring necessary to make the subs sound good. What will have better sound quality and what will be louder overall? 1 12" Fi BTL or 2 13.5" JL w6s?