Do you have a Garand?
Ammo or a reloading press would be a good investment, too.
I've never fired a PS-90, but I want one too. So light, compact, and they seem like a blast to shoot. However, they're so expensive to feed that I don't think I could ever really enjoy it.
As for the SIG-556, I do like mine, but at the same time, I really find it hard to recommend them to someone else. They are pretty much half-assed, and there isn't much out there yet to correct it's faults. On the other hand, I've yet to year any reliability complaints on them. Mine has been 100% since day one. Very accurate, excellent trigger, with a nice recoil impulse as well.
I wouldn't commit anything to the Masada. Not yet anyway. I'll be on it as quickly as anyone. If they make it.
Can't really comment on the 6.8 SPC or the SPR.