Yes, RISC OS does have to become one entity again, but even if it were, the userbase and machine sales are still far too small to get new software development.
I'm not sure whether the Omega is more mainstream than the Iyonix, after all you can actually buy an Iyonix! However I do think that the Omega has greater potential to expand the market for various reasons, most importantly processor upgradeability, and I also get the feeling that the Omega might get more third party development. Software and hardware development for the Iyonix seems to be exclusively at Castle, whereas the Omega uses the Simtec USB stack, and could maybe even use third-party CPU upgrades from the likes of Simtec.
Of course this is all academic until MD release the Omega, but it seems all the talk at Wakefield of the machine shipping soon were simply not true.