Sensitivity is a representation of efficiency. The higher the efficiency, the better the sound quality.
Xmax is a measure of the "throw of the cone peak to peak. This is a direct representation of the volume of air the subwoofer will displace..
A High X.Max subwoofer will displace more air - and therefore produce more SPL in a vented enclosure.
Sensitivity on these subs aren't crucial, because the customer is looking for LOUD and will purchase a large class D Mono amplifier to power it - characteristically low in sensitivity as well due to the way the power is inverted. Highly efficient, yet low sensitiviy.
I am definately a basshead! My show car has (4) 15" Cerwin Vega Stroker PROs. (one of the most highly EFFICIENT high xmax subs available) (86dB). (36-44mm Xmax (adjustable for SQ or SPL)
If you are looking for SPL, High XMax, Low sensitivity, and lots of power is the way to go. Generally, to increas SPL by 3dB, yuo must either increase surface area of driver 2x or double the input power. Slot ports, aeroports, and loaded horn ports can also increase your dB level by up to 3dB as well.
Properly constructed enclosures have an effect as well. Instead of wasting some of your power on vibrating the enclosure, it should be braced properly to reduce the wasted energy.
All of these tips will add tou your satisfaction of the product. You can make a $100 sub sound phenomenal in a proper enclosure, and a $400 sub sound like $hit in a prefab or poorly constructed one.
Your original question - with all things equal except the subwoofer, the subwoofer with the higher Xmax should yield the most SPL - due only to the fact of air displacement.
If you have additional questions or need clarification dont hesitate to ask.