If you could get a good price on the Haitink set, i think it is quite a good one. I often see it going for $58 or so and up, which is too high, IMO.
Haitink's early Mahler is much better than his later efforts, IMO. The original RCO set is very well recorded, and you can hear details very well. However, it is very light in the bass department, so the works do not quite have the oomph they should. The 7th in that set is quite good, IMO. I don't think there is a real dud in the set, though many complain that Haitink does not do a good 8th. I wouldn't know because I do not know the 8th well enough to know a good one from an OK one.