The obvious visual difference is the body position. The upright exercise bike is as it sounds and is more in line with what you would experience on a normal street bike and the pedalling action is basically the same. From this position you drive through your legs and hips to turn the cranks and make the bike move. On the recumbent exercise bike, you are in a reclined position with your hips, buttocks and lower back supported by the seat. From this position you drive through your legs to pedal.
So, apart from the more comfortable seat, what is the difference?
As you would expect, the main difference comes from your body position. When you are sitting upright, you are using your whole lower body to turn the pedals. You're using your thighs, glutes, hamstrings and calves in concert giving you a much more economical movement because the load is spread over many different muscles. When you are reclining as with a recumbent bike, your hips are less mobile which means that your glutes (or butt muscles) are less involved in the movement which places more load on the thighs and calves and may also make it harder to utilise your hamstrings as well.
The advantage of the recumbent position shows through in cases where people have lower back or shoulder issues as there is less load on these areas whilst pedalling. This enables you to get a low impact cardio workout without compromising your spinal posture.
Both types of exercise bike come with options for increasing workload and adding interval training so your choice really comes down to what your personal requirements are. If you are just looking for an indoor bike for general fitness then either one will do the job. However, if you want to get out on the road eventually then the upright will be far better for you.
I always tell my clients that in the early days of a home based exercise programme it is better to choose options that are simple, easy and comfortable so that you are more likely to keep going and if that means you start off with a recumbent bike then go for it.
Enjoy the ride!