Whisper:
The higher the spin the dryer your closes will be. 1400 spin is about average - anything higher than that can technically damage more delicate clothes (unless you go for one that has a delicate cycle).
Also, Washer/Dryers do give you a break down on their efficiency ratings: energy, wash, spin & dry cycles... The higher the energy rating the less you'll pay in the long run to actually run it, however it'll cost you more upfront for the higher energy ratings...
A bit of a catch-22 especially on washer-dryers as they're combi units (because of the dryers - they're not really energy friendly) - its not so bad on washing machines themselves. Any machine worth its salt will usually give you a breakdown on what the energy consumption is to run it as just a washer & a combi unit - this'll help give a more accurate estimate as to how much it'll really cost you to run it.
Any machine that has a noise rating of 55 or below is considered to be a "quiet" machine. You'll really notice the difference if you had an older machine from about 3 years ago and beyond compared to a quiet machine. Plus, they don't rock near as much!
To get the most accurate information take a look at the manufacturer's website itself before making any big decisions.
Hope that helps.