Most of the "boomy" complaints of ported systems isn't the box itself, but the boosted frequency response at lower frequencies. Ported has a relatively flat response down to tuning, say in the 30hz region or wherever you decide to tune the box, and below that rolls off sharply at a rate of 24db/octave. Sealed has much higher point of resonance, in the 50-60hz region typically with most 10-15" subs, and rolls off more gradually at a rate of 12db/octave below that.
Due to cars being so small, they have cabin gain, effectively it is compression due to the cabin itself being so much smaller than the wavelength of the frequency produced by the driver. Notice how much stronger the lowend of a car is compared to a home, this is because as frequencies get lower, the compression of the small car environment boosts frequency response. the rolloff of a sealed box tends to give a flatter frequency response because the boost created by the cabin gain tends to compliment the rolloff of the sealed box, creating a relatively flat frequency response. Whereas because ported will be flat to such a lower frequency, the cabin gain will boost that creating a bolstered output in that region, which some prefer, and others call "boomy" because it has more output relative to the upper subbass and midbass region.
The easy fix: EQ down the frequencies that ported boxes are stronger at, effectively equalize the output to mimic that of a sealed box, or basically EQ the response to give the optimal response in a car regardless of the enclosure you use. No, that doesn't negate the purpose of having a ported box for a couple of reasons: A port actually doesn't perform like a leaky hole in a box, it's a tuned mass of air that acts like a solid diaphragm. When you produce frequencies around or at tuning, that tuned mass of air is actually sucking energy out of the air pressure created in the enclosure. The port basically uses the cone of the subwoofer as the fulcrum to bear the force of its own acceleration. Doing this it loads the driver, which limits the subwoofer's excursion (usually around 1/4 the excursion that the same sub in a sealed box would produce at that frequency). That reduces distortion and at the same time gives you the higher output that ported boxes are known for because both the driver and port are contributing to produce the soundwave. When you go that much further and EQ down that point, you lower excursion that much more, you also use substantially less power output to get that level of SPL (stressing amplifiers less and keeping them from clipping at high output), and also gives you more effective headroom because you are nowhere near stressing the mechanical limits of the subwoofer, meaning you can push the sub harder because you don't have to worry about it bottoming out at low frequencies. It's literally a win-win situation.
In short, I vote ported :thumb: WinISD, as mentioned by brwillms, is a very good program if you want to model up the subwoofer and see what will work. WinISD Pro alpha (they have an Pro alpha and the older, simpler beta version of the original, both are free) has much more and can actually let you compare all aspects of the enclosures, and you can see the difference you'd get between sealed and ported for your specific sub.
