As a pilot of old 747s, I have flown the 3 different engines. A pilot point of view -
xxx
The JT9s have the reputation of being very easy to maintain, and are known to be able to sustain bad treatment (from pilots like me) for a long time before requiring overhaul. The CF6 is, like our friend 16down says, a pilot's engine, it is my favorite when it comes to handling the thrust levers, although I personally prefer EPR to N1 as means to set power. I liked the RB211 as well, but it is often a maintenance nightmare.
xxx
I remember my airline having leased a 747-238B (ex Qantas) for a few years, the engines were (supposedly) JT9D-7J, but actually, one of the engines was a JT9D-7A and this was not known by the crews (including myself). We often operated that -7A at 7J power (EPR) settings, and it survived the bad treatment for well over a year or two, when it was discovered it was actually a -7A and was soon replaced by a proper -7J.
xxx
In short, JT9s can suffer gravel and birds with little worry for FOD, which is not the reputation of the CF6s and certainly not for the the RB211s. Our director of maintenance jokes in saying that with a RB211, to expect damage if ingesting little butterflies...
xxx
Nice peace of technology, that RB211, and a pain to study in the classroom.
xxx
Happy contrails