Hi, I have booked a SS on the infinity - I too read about whirlpools, but there isn't one on the milleniium class of ships. I do believe the cabins are all similarly sized, but that in some cases the balconies may be slightly larger if you get one of the triangular shaped ones. Afraid, I can't comment about the walk-in closets.
Most people I have corresponded with speak highly of them, plus you get the butler service with them which is a bonus.
With regards to views - I have not read anything to suggest bad views. Hope this helps a little
We have had sky suites on the Millenium and the Infinity. Our balcony on the millie was mid ship and very private. The butler was a nice treat also. Brings tea and crumpets etc in the afternoon and is very helpful for whatever you may need. There are no whirlpool tubs in the suites.
Our room on the infinity had a large balcony. Can't remember the cabin number, but it was near the elevator at the back of the ship and had a balcony that jutted out. Great view, but also was exposed to balcony right next door. We really liked this room and never heard any noise from the elevator. Was a quick trip up for coffee in the morning.
Hope that helps. I've been tempted to try one of the rear balcony rooms one day. They look real neat and others that have used them have really enjoyed them.