Hey David,
This is very difficult to say, even for the manufacturer, since it depends on what you plan to do and which tools you want to use.
Generally the thicker the tool you use, the more force you can apply to the tool before it bends or vibrates, which translates to: the higher the speed with which you can remove material.
The Onefinity is a sturdy and rigid machine wich allows for high material removal rates. However most people here want to mill wood, where that matters. You said you want to machine clock dials in brass sheet. Is this rather fine work, how thick are they? What are the tools you intend to use? And the question is, do you want to exclusively machine brass sheets, or do you want to mill wood or other things too?
One thing that sometimes matters when milling metal where you want coolant to reach the endmill, is if your collet nut is very big and you use a very small endmill and you go into small pockets. Then the collet nut can be in the way, as this person describes with their ER32 collet. But they talk of a ER32 OD approx. 2" (50mm)(!) large collet nut. However the difference between ER16 OD approx. 1.25" (32 mm) and ER20 approx. 1.33" (34 mm) collet nuts is only slight. A difference would be there if you compare it to ER11 which is only approx. 3/4" (19mm) diameter. By the way, Mechatron offers a 80 mm ER11 model too.
Here a few things generally to consider:
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ER16 collets can take tool shanks from 1-10 mm, which would mean up to three eights inch tool shanks, whereas ER20 collets can take tool shanks from 1-13 mm, which would mean up to half-inch tool shanks.
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All tools that go into a ER16 collet can also go into a collet for ER20 receptacle, but not the opposite. However you have to judge yourself on whether you will use tool shanks larger than 3/8" / 10 mm.
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Also one should take into consideration that when using tools with a larger shank, the thicker the shank, the stiffier the tool and so the longer the tools you can use – and thus the deeper the pockets you can mill into your workpiece.
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We also asked Mechatron too what are differences between ER16 model and ER20 model, what they answered was:
The ER20 variant is somewhat stiffer, since larger bearings are installed.
Larger bearings means longer life and less heat.
A larger shaft diameter means more stiffness and more resistance against vibrations (chatter).