Have several thousand hours on various Gulfstream models, most recently on the G550 and now the '650 and I would agree with Flexjet that either Bombardier or Gulfstream models are equally reliable nowadays. I would also agree that Gulfstreams are just like Globals in that they don't like being left out in the cold, I always noticed on the 550 that the cabin systems could become quite ill tempered after a night or two outside on a cold ramp.
That being said, I've always been impressed with the dispatch reliability of the Gulfstreams, parts may break, but the MEL or CDL always seems to let you finish the trip without interruption and give you time to have the part waiting for you at the next maintenance base.
My experience with the new 650 has been somewhat mixed so far, like most operators of the type I am very impressed with the performance, range and handling of the aircraft and despite some software quirks the flight operation systems of the aircraft are solid, the cabin systems however are still a "Work in progress". I should note that this experience is based on A/C serial numbers 38, 63 and 64, I would expect that customers receiving serial numbers 100 and above should receive a product that is much more reliable.
When considering which product is best for your client it really comes down to which model best suits your mission needs, to which I will add one more observation and that is that Gulfstream has been shown to consistently "under promise" and "over deliver" when it comes to performance specifications.