There is definitely a huge difference between what a hotel does when a Diamond books a regular room (i.e. supposed to put them into the best available regular room) compared to when a Diamond books a suite (IME most hotels try to put you into their crummiest suite). Often, a hotel's best regular room is far better than the crummiest suite. In that respect, using a "Diamond Suite Upgrade" is actually a DOWNgrade.

I've not been very lucky using Diamond suite upgrades. On two occasions I had a confirmed upgrade but it was not available at checkin so I was busted back down to a regular room. It's a bit irksome because they don't say "we're sorry but your confirmed suite upgrade is not available". They just matter-of-factly say "it's not available". We can conclude that there's no sorrow involved. The benefit was promised, reserved, booked ... and not available.

I'm a new Diamond this year. On -every- occasion when I used a suite upgrade, I was seriously disappointed. Perhaps I expect too much. One occasion had serious problems with the room (it was raining indoors). In ALL the other occasions I end up in room with very restricted views (i.e. looking at a skyscraper across the street and/or had a room with NO outside view whatsoever) and/or weird floor plans (room layout is so cool and trendy and cramped that there's actually nowhere to sit or even stand except on the bed).

On many occasions I did NOT use an upgrade certificate yet was upgraded to a suite anyway. Perhaps it's because the surprise was unexpected but in every case those suites were -fantastic-.

In summary, IME when you use a suite upgrade you get a disappointing room (if they even honor the upgrade at all). AFAICT when you book a regular room you "normally" get their best regular room, and when you book a suite you "normally" get their crummiest suite.