A word of warning. The links state this, but it might be worth emphasizing it here.
A 16mm fisheye will not be circular on a small sensor digital camera. You will only get the full effect with a film camera or full sized digital sensor camera.
If you are looking for something with a more pronounced fisheye effect, then go for the Pentax fisheye zoom. It isn't circular, but at 11mm, will definitely give you that fisheye effect, not to mention it will be 180 degree field of view. You also get up to 17mm, which allows you to dial in the amount of distortion you want.
An 8mm Peleng circular fisheye would be best on a small digital sensor camera if your trying to maximize your circular effect.
Experiences with the Peleng 3.5/8 mm Fisheye