Well, if the track record of completely new machines is so important, then surely MicroDigital should have an advantage over Castle? But personally I don't think it is relevant.
The Omega has had a number of upgrades through firmware updates, wich, among other things, added new features and increased performance of the FGPAs.
And as for "more ways to upgrade the machine" in the form of daughter-boards: The Omega offers a high-performance bus wich can be used for all sorts of things, among them a replacement or complementary processor, like the planned Xscale and FPA. And very little (if any at all) glue logic would be needed on the daughter-board, as the northbridge can be altered to offer an optimised interface. So the Omega offers at least two ways to upgrade the machine, in addition to the PCI interface and simple firmware updates.