This depends on your preferences. A spin bike is more like a regular bicycle - you control the tension and have to manually adjust it according to the workout you want to do and the seats are normally pretty small. An exercise bike usually has more bells and whistles - you can set programs and the tension will adjust automatically and they usually have more 'cushy' seats. That being said, I prefer simple, so I usually use the spin bikes when I'm at the gym and you can buy a gel seat if you want to make the seat more comfortable.