Congrats on your purchase. Sad you didn't go with the 1237, but if your 621 is in good shape, it looks like a good deal. Externally, it looks a lot like my 1237. I think the tonearm is exactly the same.
As I mentioned in the other thread, I had tried a high compliance P-mount cartridge (with adapter) and it didn't perform very well. I think the effective mass of these tonearms is too high. The cartridge that does work well for me is an AT11E, which you can't buy new anymore (you can find used ones sometimes on ebay). Since I can't find effective mass specs for the Dual tonearm, nor compliance specs for the AT11E, it's hard to say for sure. My guess would be to stay below 15.
The AT120E/T might be a good choice. I purchased one recently, but haven't had time to check it out on my 1237, but probably will eventually.