If you are used to having brisk walks in good weather then I would suggest you get a treadmill and then you can continue to walk even when the weather is bad. I use one all winter as here in Canada the streets and roads, being covered with ice or snow, do not invite one to walk. I did have an exercise bike at one time but found that it really didn't keep me as trim as walking does and so I went the extra cash and bought the treadmill and never regretted it. I have a lung problem and so need to keep my weight at a certain level in order to facilitate my breathing. If you are set on getting an exercise bike then by all means do so, but I prefer the treadmill.
Good luck with with ever you decide.