FF,
Which one makes you work harder? Not sure.
If the question is do you work harder in a spin class than at home on the turbo then it depends on whether you are trained to train and know how to push yourself until it hurts or if you need the instructor to shout at you. Note: I have never been sick doing intervals in run training but have been sick doing intervals in rugby training - there is nothing like a big scary man shouting at you (except Melds which is scarier).
If the question is should you have a spin bike at home or a turbo then I would probably go with a turbo, unless you can afford a wattbike. It comes down to measurement. Spin bikes have no measure of progress, you can work out how hard you are working from HR but you can't work out if you are progressing. A turbo lets you measure distance, a gradient proxy and HR. Gradient proxy is lever on a set level or watt output etc. They are never accurate but accuracy is over-rated. As long as they are consistent accuracy doesn't matter so much unless you have 4 houses wit ha different turbo in each.
So the speed (distance/time comp) at a fixed gradient proxy for a given HR will indicate progress.
That said a Spin bike with Spinervals is enough for a hard workout if you can push yourself. I have Spinervals and a turbo with PC link. I can choose Spinervals, virtual courses, video course and pre-programmed interval/pyramid sessions (Tacx Catalyst).
Spinervals and video courses get expensive fast whereas the catalyst pre-programmed courses has 40+ courses which provides lots of variety. I have Flow with i-flow upgrade.
M..eface