I have never used the Ironmasters, but I have owned a pair of Powerblocks for a year or so, and I can't say enough good things about them!
They are holding up really well, and the time it takes to change their weight is REALLY fast.
That was my main concern with the Ironmaster, it looked like they would take some time to change 'em out, and I don't really want to fuck around like that while I'm lifting if I don't have to.