It really depends on how long you plan to stay at the hotel. For three months or less, I suggest you book a suite (I recommend the two bedroom suite) at advanced purchase rate (or corporate rate if you have one). Either that or purchase basic room and upgrade using your Diamond suite upgrade + points.

The hotel does offer a long-stay suite rate, but Diamond benefits do not apply. Also, you cannot credit the stay to your Goldpassport.

Considering the benefits that come with Diamond membership (i.e. access to club lounge, free internet, harbour view, high floors, access to gym + pool and DVD library, Goldpassport points and promotions), I believe it is more beneficial to just book the room via the hotel website.