I am a Diamond with Hyatt and a platinum with SPG and I absolutely cannot choose between the two. I have enough nights and stays each year to renew my status with both.
I love Hyatt's free breakfasts as a Diamond - and the Regency clubs. I love calling customer service and diamond reservations because they move quickly on my behalf. There's no wasting of time. With SPG, I dread calling the Platinum line. The answers seem different every time.
Hyatt has some spectacular properties; and SPG's availability of awards and quality of properties is hard to beat. I know that with double points, I can book an award at nearly any Hyatt without capacity controls, and I guess given the earning differentials, it might even out. But the perception is there that it's much easier to get awards at the most desirable locations with SPG over Hyatt.
If we could merge the best of both programs? WOW.