In short,
Mid-tier: Hilton
Top-tier: Hyatt
Hilton Gold status is, by far, the most convenient for me. Upgrades to the executive club level and free breakfasts reign the day. More often than not, I'm upgraded to a larger or corner room and on occasion a "junior suite" type room. Have not received any real suite upgades as a Gold. Also have free access to hotel gyms. At Hilton, I have received nothing but great customer service -- both on the phone and at hotels.
Hyatt blows the competition away at its top tier Diamond level. Welcome amenities are generous and I have never been denied a suite upgrade when requested (yet). Like HH Gold, Diamonds receive complimentary breakfasts either on the club level or, if this is not available, in the hotel restaurant. Hyatt staff consistently go out of their way to please me. I have been surprised by their customer service
initiation (vs. me asking for stuff). At the current rate, thought, I'll probably only make Platinum this year and will severly miss the great Diamond treatment.