Seeing that IH has tweaked my memory a little and I have to amend my earlier reply. I could be wrong, but I believe the older two-digit IH series like that one used a different knotter than the later All-Twine ones used on balers like the 435, 440, New Ideas, etc. Once again I could be wrong, but I think the older IH's like that 50T used the old Deering design that was a slightly modified version of the ones used on the Deering grain binders. I believe the All-Twine knotters they later used were a beefed-up version of the McCormick knotter design. There's a chap near me who rebuilds and sells a lot of grain binders to the Amish & Mennonites, and he told me the Deering knotter style was less persnickety than the McCormick ones and is thus the preferred knotter for the Amish he sells to. I don't know all the differences, but I know the Deering knotters had a stripper to pull the twine off the billhook (similar to the Rasspe knotters) while the McCormick/All-Twine knotters didn't (they relied on the bale being pushed out of the chamber to pull the twine off), The lack of a stripper was one of the primary sources of frustration with the later All-Twine knotters: If there was a little slop in the linkages or if you didn't keep the revs up, they weren't happy at releasing the twine.
If my memory's correct in all that, the Deering knotters on that 50T a might be more reliable than the later IH's, but I suspect parts are nearly impossible to find for them.
If you're looking for a reliable baler that you don't have to do much tinkering on, can readily find parts for, and can count on to punch out a few hundred bales a day for several days a year without a fuss, then I'd stay away from the IH. But seeing the nice-looking barn and shop in the background of your NH picture and hearing that you're already dealing with the wackiness and unconventionality of engine-driven balers, I suspect you must like to tinker and rebuild old/unconventional equipment. And you probably don't rely solely on your square baler to feed your animals. In this case and if you can get the IH cheap enough (maybe try to talk him down a little), no harm in buying the IH to be another toy/tinkering project.
But if reliability, consistency, and parts availability are the primary goals, it'd still have to be the NH for me.