My vote may not hold much weight but my honest opinion is piu.  Unless you can find a more recent ddr mix which are eamuse only so not happening.  X and x2 suck.  X3 is good but outdated.  Piu is the easier to acquire.  I bought my whole cab for less than 200$ and have recently seen a pro 2 and fiesta 2 for sale in the 300-500$ range.   

If you've already got a DDR machine, I'd say go for a PIU machine to add variation. PIU machines may not earn quite as much as DDR, but for the few players who still want to play PIU, that would be good to have. I've heard there's only like 1 PIU in the pacific northwest and that's in Seattle. All the others there used to be have all disappeared, so PIU would be ideal for serving the minority who still want to play it. DDR is far more common and most of those are DDR Extreme, but if you have that 4th Mix and could possibly get a DDR 3rd Mix that would be good for a lot of us nostalgia freaks. ITG2 is pretty good for its customizability, but there are already a few of those around.

I'd say Pump, simply because there are many more ddr cabs than Pump ones in the vicinity.