I have been a fan of both these stallions for many years, but it is my understanding that De Niro tends to stamp his foals a bit better than Don Schufro. While there are always exceptions to every rule, the De Niro offspring show a certain consistency in type and topline (and often in color and markings, with so many of them chestnuts with blazes and some white on the legs). The Don Schufro offspring are sometimes a bit all over the map - some are large and a bit “rough hewn” looking, others are quite lanky and almost greyhound looking (and sizes can vary, from smallish to very tall).
It has also been said to me that De Niro very frequently puts on a very nice head/expression - not a tiny, super elegant and refined head by any means, but a handsome “warmblood” head with a large, dark, and intelligent-looking eye. Don Schufro offspring, by contrast, can have a somewhat large and almost old fashioned looking head.
That said, I would love to own broodmares by both stallions.
And yes, the DS frozen hasn’t been as reliable as the DN frozen, and, as mentioned above, his fresh semen isn’t great anymore, either. He is 20 years old this year (as is De Niro), but DS only has one testicle (lost the other one quite some years back due to inguinal hernia).