If you already have a decent R8 set, there's no reason not to use them. I have an ER40 chuck installed in the Bridgeport as I have a larger selection of good quality ER40 and the chuck was a similar cost compared to filling in the gaps in my R8s with quality collets. I have some end mills that I lack R8 in the correct size for, so I usually leave the chuck in there and use the ER40s. I also already have the ER40s and a chuck for the lathe, which gets a lot of use. My collet blocks also use ER40 for the through hole capability as another post already mentioned.
Rather than using the drawbar, you can change collets using a spanner on the ER chuck. Some people like this better, it's about even in my mind. I'm about 6'4", so the drawbar nut on the top of the machine isn't a big deal to reach up for. That could be a bigger issue for the more vertically challenged. A down side is you need more torque on an ER collet than you do with the R8.
While the chuck does add some runout, it's a couple tenths in my case. That is acceptable for the work I do. If it ever isn't, I won't hesitate to get more R8s as needed.
I don't notice any difference in rigidity, though I suspect it's there. I may just have not pushed things hard enough to notice. More stickout almost has to drop some rigidity.
You do lose some space in Z as well, a couple inches. This isn't an issue for me, but I have a fairly large Z travel. It might or might not matter for you.
If you were to use the ER collets, you would need to purchase ER collects AND an ER to R8 adapter chuck. In your case, I would say not to spend the money unless you were already buying the ER collets for another reason, such as the lathe or collet blocks. At that point, it might be worthwhile to have another option for tool holding in the spindle.
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Hi Woodguy,
I'll try to answer and hope I don't add to the confusion.
I use 4 different collet systems in my shop: R8, 5C, MT3 and ER40.
R8: The most common (by far) taper collet used for milling machines in North America. Thus, the tooling is readily available and affordable in the new and second hand markets. I've never seen an R8 taper on anything but a mill. The collets require a draw bar to pull them up tight so they are closed end. The collet is released by backing off the draw bar and tapping it with a dead blow hammer.
5C: Quite common on production sized lathes, this is a self-ejecting pass-through collet. Though similar in appearance to an R8 the body is larger and the taper is slightly sharper and more abrupt. The outside far end of the collet is threaded for a draw tube that is connected to the locking/ejecting mechanism.
MT3/4: These are Morse Taper collets. The taper is rather shallow and locks quite firmly using a draw bar. They are closed end. Releasing the collet is similar to the R8 but may require a rather sharp smack to release as they can grip very tight because of the slight taper.
ER series: All of the ER series are similar in design, the difference is the physical size of the collet and capacity. These are a pass-though and self-ejecting design. The collet itself has two tapers and is shaped somewhat like a truncated diamond. A nut with a ring in it locks into a groove in the collet, the nut screws onto the chuck pressing the collet into the taper. The collets for each "step" (ER11, ER16, ER32, ER40 etc) are not interchangeable with each other.
The R8, 5C and MT collets have very little tolerance for off sized work. Your material/tool must match closely the size of the hole in the collet. The ER series is much more forgiving in this regard allowing as much as a millimeter variance in size.
There are many more collet systems used throughout history and the world but these are the most common, you will be able to find any of these at reasonable cost at any of the major tool vendors or on Craig's List/ebay.
For more information, please visit Sealed Er Collets.