Well, it's nice to have the choice of two new and fast 'native' machines, non? If you want an ARM-based solution, then either an Iyonix or an Omega seems like a good choice to me; which of these one plumps for is a very personal decision.
I opted for an Omega because I wanted an up-to-date machine that was compatible with *everything* that I use on my RiscPC -- MicroDigital billed it as a 'go-faster RiscPC', and that is a good way to look at it. All software that is RISC OS 4 compatible will run on it, and (unlike Iyonix) it is capable of working with parallel port devices such as scanners, zip drives, sound digitisers, etc. (all of these were relevant in my case, but may not be so for everyone).
Compatibility is, I feel, the key advantage: if you want to use certain applications, i.e. Sibelius, Topmodel 2, CFS, most RISC OS games, Select, etc., or you want to run 26bit apps natively, then Omega is currently the one to go for (though this may change as Aemulor is enhanced). And, of course, as and when applications are updated for the Iyonix (i.e. Impression), Omega users will also benefit. One of the first software upgrades that I purchased for my Omega was ArtWorks 2 which is simply gorgeous!
Whichever machine you choose, be positive, be happy it's RISC OS, and enjoy