IHG tends to be VERY easy to get platinum at. As a matter of fact I did it in 7 nights stayed and there are people on that forum that have done it in less. The one problem I have with IHG when booking travel is that booking a stay at an IC in SE Asia (where other programs tend to be VERY cheap) is the same as booking a stay in a place like Tahiti where all hotels charge a lot of points. I would agree with the above post saying find out where you want to travel or what other things you want to do with your points then choose your program.

Some quick thoughts:
Marriott has no hotels in the Pacific other then Hawaii, but has those nice 7 nights + airline miles travel packages.
IHG has no properties worth staying at in Hawaii (there is a holiday inn in Waikiki that is very uninspiring), and IMHO has the worst Plat benefits.
Hilton has some cool options for purchasing theme park tickets (My wife and family are HUGE Disney fans).

Again, decide what you want to use the points for and choose the program that best allows you to do that.