Upright Exercise Bike vs Spin Bike

[Sunset Dragon]

I have a problem with my knee and my leg muscles need to be strengthened. At this time, I am unable to use weights or perform squats. It has been suggested that I either swim or use an exercise bike to maintain fitness.

I am interested in purchasing an exercise bike primarily for the purpose of strengthening my legs. For those who have used both kinds of exercise bikes mentioned in the title, would you recommend one over the other?

I read that the spin bike is great for cardio, but I also read that once you gain momentum, the wheel can spin without you needing to force the pedals to move. I read that the pedals on the upright bike stop moving the moment that you do, therefore requiring constant effort. If this is the case, it sounds like the upright bike with resistance would be better for me to build leg strength versus the spin bike. However, I don't know if this is true and was wondering if anybody has any thoughts about it.

Thanks!