When I was going through the same thought processes while looking at IST choices, it boiled down to this:

Grand Hyatt - Cheaper, but really an average "could be anywhere in the world" property with very little charm, and quite dated as well.
Park Hyatt - Typically 30% or more expensive, but considered by many to be one of the top 10 Park Hyatts in the world.

When I eventually do go, I will either be staying at PH or a different chain altogether - the GH honestly just doesn't appeal to me, unless there is a whole side of it that no-one has ever written about before.