That's interesting Matt because the guys we raise seed wheat with traded their sdx in on a p500 and stand relusts were excellent in wheat. Almost all non airseeders lack in furrow firming unless they have Keaton's. To me the consistent depth control of the parallel link far offsets the lack of firming wheels, but our conditions are different out in western kansas, and we seed over terraces. My vote is for the case, better design then the deere. If you really like the deere cart run it on a pd500 tool bar.
The P500 may well have been an improvement for your neighbors, since the SDX had so many issues (and most of them had the seed-lock wheels omitted in later years). Not to mention luck in any given year in getting a stand -- maybe they had more moisture for their P-500 than they did when they had their SDX. In most aspects, the P-500 fixes a lot of the silliness in the SDX design, but the omission of a seed-lock wheel was a really bad idea for selling them in USA. Not to mention that their packer wheel doesn't close the furrow most of the time, not even if it has 150 lbs of pressure on it -- Case does admit they have a problem here.
Depth control may be irrelevant if you don't press the seed into moisture, and close the furrow in an acceptable way.
There are hundreds of Deere 50/60/90 drills that go over terraces throughout Ks, and the terraces in the eastern half the state are waaayyy nastier to seed over. Granted, there may be skips, because that opener has almost no downward travel. But most of the time it is good enough to not bother with seeding the direction of the terraces.
To say that the P-500 is "a better design" than the JD 50/60/90 is a bit of a stretch. In some regards, yes, vastly better (residue cutting; going down into depressions). In some regards, vastly worse (seed firming). And I'm completely color-blind -- I just want the damned drills to get a top-notch stand in continuous no-till every time we go to the field. We don't yet get that from either Deere or Case, and not anyone else either.
No love for the JD carts. Flexi-coil (Case-IH) has better technology in the metering and air stream.
respectfully,