Well I'm just ahead of you on the learning curve.
My control panel design will feature both rotary joysticks and spinners (eventually - cost is high). I was told that rotary joysticks don't freely rotate like a spinner does and some games will suffer if you can't just give the control a spin and have it keep spinning. Tempest is a good example of this.
Also, there's two kinds of rotary joysticks, mechanical with 12 positions, and optical. The mechanical joysticks click as they turn. Mechanical joysticks would definately not work well as a spinner.
Cheers,
Civilan