Depends if the price difference is worth it to you or not. I'd go for the BL15 out of the two, but if I were looking at spending $350 on a sub I wouldn't be looking at a used lvl4 as my other option either.
iv heard a 15" BL in my ranger and they get loud, but iv heard a lvl 3 15 in my boys jeep and that was surprisingly loud so the 4 should be louder. BL has cone but both will be loud.