There is no statistical evidence that says 3 engines are better than two, especially when you take into account the make and model of the engines in question.
Another factor in choosing an airplane is the maintenance and support issues. Gulfstream factory support is EXCELLENT, as is their warranty program. The ability to climb higher, faster, at heavy weights is a distinct advantage, especially when operating in areas where convective weather is the norm, like the USA and certain areas of the South Pacific.