It is puzzling when one asks such a question that one does not describe the context of use and the machine in more detail. A 5C chuck would be impractical on a mini/small lathe as well as an ER40 chuck due the the weight size, and stick out. What is the budget and what is the expected use? In a larger lathe a 5C collet closure system may be more ideal and/or a hand wheel collet chuck type, much quicker than and ER system and more repeatability. If you have other machines like a mill, than an ER system collets could be used between machines. The topic has been extensively discussed in previous posts, as well as David Best's white paper.
There is no best collet system between the two, they each have their strengths and weaknesses, and it all depends on the application/use. I have both, I made my own ER40 set-tru chuck quite a few years ago, maybe have used it 2-3X for odd size metric stock. It has a stop system that threads into the back of the chuck similar to the 5C types, but neither of these threaded stop systems are that accurate vs. a through the spindle stop systems that always maintains the same distance as the collet is pulled in/compressed. I use the 5C collet system because I do a lot of small parts, small threaded cap screws and pats that I am turning to a a thickness down to less than 0.1". The 5C with a full set of 1/64th collets is more universal for my needs, but if the intent is to just hold different size stock for turning than an ER system would better holding power and require fewer collets.
Custom ER-40 Set-Tru chuck with threaded stop system. The stop system thread is the same as the 5C collet internal thread so can be used on either chuck system.
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Set-Tru 5C chuck and 4J 8" combination chuck, the latter can hold square stock down to 0.2", they each have a different work envelope.
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One of the strengths of 5C collets is the ability to easily hold very small and thin (thickness) materials. Pin below is 0.550" overall length and made 10 with a high repeatability, washer needed to be 0.1055" thickness. I used a through the spindle stop system for repeatability.
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