The Omega 18 is really tuned as a high-power midbass and not so much as a subwoofer. You can put it in a cabinet giving it an EBS alignment, and it will work below 40Hz. But the Omega 18 is really best suited for 40Hz up in high-power applications.While not being familiar with the Selenium 18SW1P, its specs indicate that it is tuned to be used lower. It will generate more output in the bottom octave at the expense of performance higher up. These kinds of optimizations are functions of the specifications of the woofer, which were designed to meet a specific goal.
The Omega 18 is more like the JBL 2241, while the Selenium 18SW1P might be more like the JBL 2245. Which is better for you really depends on what you plan to use them for.