Mountain Bike Or Exercise Bike?

I'm at the moment trying to figure out whether to buy an exercise bike for inside or just a regular mountain bike. My kids are a little older now and want to bike lots so I'm kinda leading towards the mountain bike. I've always had one it's only been in the last 10 years I haven't had one and have always enjoyed one. Exercise bikes are good and all it just might be better to purchase one once the colder weather comes back.