I have tried Strathmore 300 (pretreated with Citric acid): smooth highlights but no blacks. I should get some samples of the 500 which is supposed to be different from the 500 (I would like the 1 ply if it survives the processing).
I had a sample of the COT320 and found it quite similar to the Arches Platine and is apparently made in the same factory. COT320 may have a better quality control than the Platine. There could be some difference in sizing - surface treatment?