(from MKSJ on hobby-machinist.com)
Two camps on ER vs 5C collets for lathe material holding. Having both, I primarily use 5C for a couple of different reasons. I not a fan of having to have a wrench and a nut to tighten the work, 5C can handle much shorter stock which is what I am most frequently clamping. I use a speed handle on the 5C chuck so very quick to change collets. Downside of 5C, limited holding power vs. ER collets for longer stock, limited clamping range, really requires a 1/64th" increment 5C collet set. ER collets tend to clamp uniformly from both ends so less axial run-out the further you go from the chuck. ER collets tend to have a smaller TIR, but it all depends on the quality and the clamping range.
If you look at something like an ER-40 collet set, many vendors will sell you a 1 mm increment set and say it covers the full range. Personal experience is although they may be advertised as 1 mm clamping range, the TIR sucks at the extreme range. I use a 1/32" increment ER-40 collet set and my high accuracy ER collets are not made for a range of clamping, but a specific increment. In addition with ER systems, a higher quality clamping nut and better collets (like techniks) make a significant improvement in TIR and axial skew of the work.