The reply has to come from an answer to another question: What specifically is your friend's goal with this subwoofer system? Does he want it to make whales surface with 20Hz notes at the beach or does he want it to get loud as hell as a daily driver? I went ahead and cancelled out the notion that he might want to get the highest SPL possible because you said tuning around 30Hz.
If he wants to go with the loudest daily street beater he should not tune to 30Hz with that vented box. The ported L5s will be louder in all but two instances.....you have 200w on each L5 vs 1500w on each L7 or the ported enclosure is so ****ed huge that its hike in efficiency is cancelled out by the inability for the waves to make their way into the cabin because of the lack of space. If you build the ported box so big that nothing can actually get around it, you could very well be louder to the ear with the sealed system. If you do have the room, the ported L5s is the way to go if he wants OUTPUT. """""I do however wonder if he has the money to choose either one, why he does not just do a vented pair of L7s?'''''''''' On the same amount of power (above 300w each) the L7s will do more damage than the L5s. The box difference is nothing more than ONE MORE cut of wood and a little more size. Anybody who makes the box out to be more than that is just hyping up the value of their work.
If your pal wants subsonic performance, that normally takes power to make the displacement, since that is what it takes to get a low note loud. Either by vent, horn, passive radiator or active pistons, it all comes down to displacement. Big vented box with either one will produce this with a large vent and a ridiculous vent length. You can tune low with a smaller vent, but the output will suffer in comparison to the larger/longer one. The L7s can move more vent than the L5s can, so again I'd recommend the L7s in a vented enclosure. For the 20-30Hz stuff, you simply have to tune it with the vent length and area. If you keep either choice of ported enclosures to the following single formula for port area, then all you need to do is work with the length for the most performance in the frequency range you choose. ((( Up to 5 net cubic feet...... 16 square inches of vent area per net cubic foot of enclosure volume. ))) Good luck with your choice and your pal should be happy with either choice if the box is designed and built well.