I ride outdoors when it's not -30C (like today) and spin mostly in the winter but I also go to spinning classes on rainy days in the summer or on days when I am pressed for time but still want a good sweat and I find that the 2 (real cycling vs spinning) are really quite different.

It is easy to get used to a spinner (pay attention to bike setup) but you do develop some bad habits that take awhile to straighten out when you get back on the road. As good as workouts can be on the spinner, I find the road much more demanding.

Both are good, a good instructor, the right music...can make for a great indoor workout. Just don't think of it as cycling.