I’ve had a ProClick P50 for years and like it. We buy spiral coil at MyBinding.com that is 3:1 pitch to fit the holes the ProClick punches. (Traditional coil is a 4:1 pitch, 4 holes per inch, but the Proclick needs 3 per inch or 3:1).
The coil is SO much cheaper! For 100 of the 16mm/ 5/8″ spiral coil it costs $29. To get the same 100 spines in the Proclick’s click shut spines it costs $49. That’s $20 more!
The click closed spines come in just 3 sizes, coil has many more options and colors for a lot less.
To figure out what size coil to get (how many pages it holds) – they have a coil sizing guide in one of the tabs on the page description for the 3:1 coil. For example they suggest that a 6mm spine will hold 30 sheets of regular copy paper – that would be a 60 page double sided document. The 16mm coil holds 135 sheets of regular paper, a 270 page document. I prefer to put slightly fewer pages in than they recommend for ease of turning.
One other thing I like about the spiral coil is you can cut it to any size. So if my 5 year old wants to make a small book with paper only 3 inches wide it is no problem. 7 inches? Done. 12 inches? Yep. They can make their own nature notebooks or other things any time they want and any size from 12 inches on down.