hI,
The maim difference is the flywheel and the fact the flywheel doesn't have a free wheel the other more subtle differences are the number of adjustments and the riding position. In an exercise bike you are positioned for comfort and not performance or not in a way that you can replicate the set up of your road bike. Most spinning bikes to an adequate job of feeling like the road. there are a few new bikes on the fitness market that try to resemble a real ride feel, but the only one I have found that is truly different and makes you feel like your on the road is called the realryder.
I think you first have to decide how hard you want to work. if you just want to elevate your heart rate to 70% of max any exercise bike will work, but if you want to work around the 85-95% range you better get a spin bike.
choose a bike based on what you want not on what someone try's to sell you. if you want to work hard get a spin bike, but if you want low intencity exercise any exercise bike will work.
Sean Vert