So I use the 150mm hard the most, definitely... the square one (takes RTS-400/LS-130 papers) I use second most although it is mostly out of convenience that I have those papers around; without it, I'd just switch on the 150mm hard.
For me, the soft always has a coarse grit and I use it when finessing something by hand that needs 'hogging'.
If I had to pick one, 150mm hard.
If I had to pick two, 150mm hard and rectangular.
If you go to buy 2, you're so close to the kit that you should just get it... you get an extra block and a SysMini.