As mentioned it is pretty hard to compare really but I would suggest one factor you would need to keep in mind... detecting a target at 150km range is a useful thing but if it is stealthy and your radar guided missiles can't get a lock what are you going to do about it?
AFAIK only the French and Russian aircraft have heavy long range IR guided AAMs that could take advantage of such systems.
Obviously detection distance is totally dependent on the target... I would suggest a super cruising target would be easier to spot at range because of the altitude and friction heating.
I seem to remember that the MiG-25 was able to detect SR-71s with IRSTs at ranges greater than their radar could detect them, which was well over 150km, but not many other objects have that sort of flight profile.
The R-40TD was designed specifically for head on shots at SR-71s and potential mach 3 bombers...