I have both ER40 and 5C collets for my lathe, but I find that I use the 5C most of the time and have been slowly adding to the sizes I have on hand. It is great to be able to hold hex and square stock and those are available for the 5C. Also the expanding mandrels are handy. The collet chuck for my lathe is a Bison with D1-4 mount that direct mounts to the spindle and is very accurate. I can often take a workpiece from the lathe directly to the mill and using the Square of Hex collet blocks, quickly index a part to mill 1,2,3,4 or 6 faces on it. A 5C quick change tool post holder is on wish list.
Example, I used 5/16 square key stock, milled it in Square Holder on all four sides to end up with a 0.300" x 0.300" square stock, then transfered to lathe and turned a 0.256" spigot on the end, 3/16" long and parted the piece off. This part was then pressed into a 2" diameter washer that was cut from 1/4" plate with a 2-1/4" holes saw and cleaned up on a mandrel using the 1/4" diameter hole that was left from the holesaw. The square stock was then pressed into the washer with the arbor press and the back side was TIG welded to prevent the square bar from coming loose.
I have two MT3 to ER40 collet holder and sometimes use a collet chuck in the tailstock instead of the drill chuck. This gives a more solid grip to the shank than the drill chuck does. The other MT3 to ER40 holder has a drawbar so that it can be held in the headstock of the lathe. This gives good accuracy, but prevents holding long stock as the collet chuck gets in the way. The ER40 Collet chuck mounted to a backing plate, D1-4 is used most of the time when ER40 collets are needed for the lathe to hold odd sizes that I do not have 5C Collets for.
Milling machine, Bridgeport Clone mostly R8 as that is what the mill has, but I do have a R8 to ER40 Collet chuck if extra length is required or I need an oddball size.
My B&S #13 uses B&S #7 collets but I do not use them much.
The Darex XT3000 Drill sharpener uses DA180 collets in the countersink sharpener and have a few collets for it, 1/4, 3/8, 1/2 and at least one metric depends on what has needed to be sharpened.
Here is a site with listing of the common collets, their dimensions and the machines that use them.
http://shopswarf.orconhosting.net.nz/sindex.html
I also Have a small collection of Rivett 2S (Seneca No. 2) and South Bend 3C collets although I do not have anything that uses them.
Walter