Very tough, as I love both!
Like club said, there are 2 types of exericise bikes, the upright one, and the stretch out one. I prefer the stretch out one over the upright because I like the stretching of the legs...makes me feel like I'm getting more out of my time. I also really like that I can see my legs stretched out in front for me to see, because as I spin I can watch the muscles in my legs working and that's a great motivator...it makes me want to spin FASTER!
And I do think it would be great on your knees, since they're stretched and and the motion involved isn't strenuous on them, so you'll be able to go for a good workout on it.