Is an exercise bike good enough for keeping fit in the winter, rather than a turbo trainer
what are the drawbacks of using an
  exercise bike?



Noise (you mentioned this)
Many quickly get bored riding on an exercise bike
Promotes bad cycling form
Can be large, heavy, expensive


My favorite alternative to riding in the snow in the winter is to ride on rollers. The rollers I have used have several advantages


Not as boring
Amazing for form
Foldable space saving models exist
More like actually riding on the road
No switching out tires
Ride on bicycle


Getting comfortable with riding on the rollers took me about 2 hours and I feel that it has been a great way to stay in shape and in good form while I'm not able or willing to ride out in the weather.

Photos of my rollers:






  Is an exercise bike good enough for
  keeping fit in the winter


Yes... and no. Completely depends on what level of 'fit' you are trying to attain and how consistent and hard you work on the exercise bike. If you are just trying to keep your blood moving and stay generally fit, then an exercise bike is a great winter workout tool. If you are working towards a 206 mile - 1 day race in the summer then you will probably want to mix your exercise bike workouts with other exercises like weight lifting and yoga.


  Is an exercise bike going to be better than a turbo trainer?


Depends. Exercise bikes tend to be more upright in their posture which may make the exercising more comfortable. Pricing and space will also be something to take into consideration when looking at exercise bikes vs. trainers. If you are wanting to be better trained and acclimated to riding your bike for races or rides, then a turbo trainer will probably better suit your needs.

There is another bicycle stackexchange question and answer similar to what I have said.
I wish to get fit, therefore should I have a heavy bike?