>Cross Training>A stationary or a spin bike?
Everything you said about spin bikes is correct. I have only been on a spin bike, and my road at tri bike for the last 3 years.
Haven't ridden the stationary non spin bikes lately.
I was thinking more in terms of moving your feet in circles and adding resistance, not the feel of the ride.
The only reliable difference I see between spin bikes and conventional stationary bikes is the position, which more closely resembles a roadie's position. The gym I go to has spin bikes and conventional stationaries (and recumbent stationaries); the conventional ones have a blinkenlights control panel whereas the spin bikes have a simply tension belt and adjustment knob There are consumer-grade conventional stationary bikes that can be set up to have almost exactly the same position as a spin bike. And for that matter, you can get an old beater bike and stationary stand (which is what I've done in the past).
I have been using a bicycle trainer while healing a shin problem. I don't have access to a pool for water running and this was the easiest thing to setup. I live in the mountains where cycling on the narrow winding roads can be dangerous and I don't have a mountain bike to use on the trails. It uses different muscles as others have said, but I think it has worked to keep some aerobic condition and I haven't put weight back on like I usually do when I stop running. In fact, I have lost a little weight.