of course there are also limitations to the Omega's expandability, but they are much lower. One could turn the 33MHz PCI bus into a 66MHz one or the 133MHz memory bus into a 166MHz one, simply by reprogramming the Northbridge-FPGA.
As for a motherboard exchange: This is a possibility that is open to the Omega as well. The problem with it is that it takes time (wich many users have to pay a technician for) and costs a lot more money than a simple FPGA update. Of course the Omega's mainboard will not last for ever, but IMO it will a lot longer than the Iyonix'.
With Xscale I see it like this: If you want a RISC OS computer with an Xscale now and all your software is compatible with it, then go on, buy yourself an Iyonix. But I would remommend you to check throughly wether this is the case. I need to do multimedia-editing sometimes, so I need software like ProSound, StudioSound and CineWorks, as well as a program to record sound with the computer. Currently all these things are not possible with an Iyonix. ProSound does load with Aemulor, but as soon as you try to actually do anything with it, it crashes the whole machine. So simply testing wether a program does load is not enough. You have to test all og the functions you require from that the programs you need. And it might be, that with the next version of Aemulor it will simply stop working (even in parts).