GM, I think you are missing the point that digging LITERALLY thousands of nails and scraps of iron that exist at every park and yard in the nation is an exercise in futility. This is the reason no company sells PIs for these uses. They have VERY specific uses, but are terrible as a "general use" metal detectors.
You are new, and seeking information is NEVER a bad thing. Just don't get wrapped around the axle of the search for ultimate depth and sensitivity. 99.9% of all buried metal objects are within the reach of almost every detector made today. The ability to find the good targets AROUND the iron is the penultimate goal. (That and being able to differentiate between gold and aluminum).
But to answer this question Honestly, it wasn't that complicated of a question. If someone wanted the most sensitive detector in SOIL with no concern for discrimination what would one use The answer is the Minelab GPX5000