Now there is nothing wrong with that approach if you are just looking for an exercise bike to have at home so that you can squeeze in some exercise when it is convenient. However, if you have a specific purpose or goal in mind then there are some differences.
The upright bike is a good all-round, sturdy and, if possible, comfortable option for general fitness and rehab scenarios. The seat is generally bigger, giving you more support, and the riding position is more upright which is easier on your arms, shoulders and lower back. This riding position allows you to multi task whilst exercising so you don’t miss your netfix (not a typo) or the thrilling climax of your latest book. This bike is also better for anyone with neck or shoulder issues as you do not have to lean forward and put load on your arms and neck because your weight is all centred over the hips.
The spin bike on the other hand, is a better option for the more focused or committed athlete. These bikes are great for those that want to get ready for riding on the road or the trail as the riding setup can better replicate a road or mountain bike.
Riding resistance is not preset so you the rider have more control on how hard or easy you work. You can make more subtle adjustments to simulate riding into a headwind or up a small rise as well as dialling it right up to climb a hill or chase the lead pack.
If you are looking for a bike for rehab of a knee or hip injury then I would tend towards the spin bike due to its simpler operation and finer adjustment capability. Your riding position allows for a fuller range of motion through the joints as well as the ability to ride in reverse, either with or without load. You can also ride using only one leg to help correct any imbalances or for extra hamstring activation.
Basically, it all comes down to application. Each type of bike has advantages over the other and whilst I personally would tend toward the spin bike I wouldn’t hesitate to recommend the upright as a general fitness option.
If you are looking for a bike then ask lots of questions and, if possible, try the different types to see which is more comfortable or appropriate for your needs.
As always, ask your local fitness professional for advice on suitability and consult your doctor prior to commencing any exercise programme.