The pads for ITG and DDR are indeed identical - however, the pads for ITG and PIU (Pump it Up) are not. Since a PIU pad is more or less a DDR pad inverted, buying a four-panel pad made specifically for DDR is a bad idea.
There're two solutions, the expensive one and the laborious one. First off, about the only metal pad on the market that has consistently stood the test of time is the Cobalt Flux - they have nine panels, so they're good for both games. (However, odds are you'll have to modify the control box or wait for CF to come out with a special PIU control box to play PIU at home.) The only minor problem you may have is that, since it's designed for both games, the corner arrows aren't expanded like on a PIU arcade stage. All that for a mere 300 scripts.
On the other hand, like several other people, I have built my own DDR pad and it has lasted for several months. As well, since I can access the parts easily, it's simple to repair if necessary. There are a few designs for homebuilt pads that feature nine panels, or you can build a DDR pad and a PIU pad with correct panel sizes. (It's significantly cheaper to build two pads if you buy the materials from a fabricator in bulk.) This approach should cost from $100-$200 depending on materials and tools used and tends to take 10-30 hours of work. (Which can be spread out over days to months.)
For more information, see the Cobalt Flux or My Home Built Metal DDR Pad threads.
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