All great questions.
I have been Diamond off and on for 6 out of the past 10 years now, and I spend 60+ nights in Hilton properties annually. I really don't see much of a difference in treatment from property to property. To be sure, there are some places around the world that are truly special, but by and large, if you are Diamond, you are special and they always let you know you are appreciated. I have been on the road for 30 years, so I am an experienced (patient) traveler, not anal like some you hear on FT.
I don't remember much special about being Gold during the off years however, so yes, Diamond is really worth it.
A few years ago when there was a Starwood property in a town I visited a lot, I got hooked on SPG and really liked them.... but then the Sheraton I frequented was sold and left Starwood, so I re-focused on Hilton.
As for other chains, I have been a Hilton loyalist for so long, I am only this year beginning to branch out again, because my firm recently inked a deal with Mariott in a city I travel a lot too. So I am going being exploring that chain quite a bit during the next 10 months. Just apped for a Chase Marriott card tonight actually.
One of the main things in picking a program is where you are going for work, and where you want to vacation. If you are in Asia a lot, then you should research the cities and cool hotels there and join the program that works best . You might find that Hyatt or SPG works for you; but from a sheer numerical point of view, Hilton has so many locations and a relatively low stay total for Diamond, it seems to be a very sensible choice to me.
YMMV of course.
