As someone who not only has owned single driver and 1.5 way Omega speakers but who can also compare them side by side, I can offer real world opinion. JLM's input may be theoretically interesting but is in no way relevant. I will also add that JLM has no way to to give an informed opinion about the sound of any currently available Omega.
I own a pair of Super 3 HO monitors and an original Super 3 (wide baffle, RS5). I previously owned a pair of Super 3U's, which are the Super 3 HO with a single driver (effectively). My Super 3's are almost the same cabinet volume, so we are really comparing two speakers that are very close.
The 1.5 way configuration will give an additional 3dB in bass using a tube amp below the BSC frequency. It really is a helper driver and works best when not in the extreme nearfield. However, with a SS amp, the bass response is +6dB, which is overpowering in the midbass and sounds muddy. With the single inductor that Louis uses, a tube amp is essential. However, if you really wanted to use a SS amp, you could use a lower BSC frequency to lower the bass boost by 3dB. In all cases, the power handling and dynamics are higher.
The single driver is only slightly thinner in the bass, and with tube amps does really well. It has considerably less power handling (feels like 5 or 6 watts) but is much better in the nearfield.