I've been up in Darwin in Jan/Feb a couple of times, and it's been wet, hot and stormy, but bearable as long as you're not out in the sun for huge amounts.
This year the wet season started late, and Dec was actually quite dry. The rains really started at the end of Jan from my understanding of conversations I had with locals.
I've been to the SkyCity casino, and wasn't impressed. Havn't checked out the hotel, but based on it's location don't really think it's anything special. It's not near the CBD, and really to me is pretty much in the middle of nowhere. (although only about ~$10 via cab to town I think).
Any reason you've chosen Darwin for a week in Dec?