For the largest possible participation, I would suggest SSD, if only because oXigen would require an investment of no small amount by groups that aren't already prepared for it. As for running SSD and oXigen in the same "race" I would say that it's not unimaginable that some would feel that the *other* system is either a distinct advantage, or disadvantage, and not want to share the field with it for that reason alone. A lot of things can be equalized between the systems, but the way the simulations work, as well as the degree of control provided by O2, may be enough of a difference, all other things being made to be equal, to disqualify it.
Worth some discussion, though. If my group participates, it would probably be SSD, though we could go hybrid O2.