A scroll chuck is quicker to chuck up and it is easier to do repeat turning as opposed to a 4J independent. But many machinist just use a 4J and can dial down the TIR very quickly. Usually takes me 1-2 minutes on the 4J, but if you do a lot of repeat work it is a bit of a PTA. TIR is relative, if you are just turning and parting the piece off than it should not be a problem, but if you rechuck or do repeat work, then it is more important.
I like Ken's method of tweaking the TIR on the chuck, providing you have the repeatability and linearity in the scroll TIR. The TIR can change a bit over the clamping range of the scroll chuck. I am going to pull out one of my old Chinese chucks that came with my machine and use his suggestion to get the TIR down.
If you look at the specs for most chucks in the 6" range, even for high end chucks, it is usually a maximum TIR in the 0.003-0.004" range, generic Chinese chucks probably 0.006-0.008". Part of this is tweaking the back plate on your machine, and as Ken mentioned, you can work the TIR down a bit. As far as chucks, really a wide range in quality and pricing, buying used can be hit or miss unless you can check it first. If you need to get the repeatable TIR down to under 0.001" then you may consider a set-tru (adjustable TIR type) chuck. You might look at some of Shar's new line of chucks, which claim a 0.0012" or better TIR on their 6.3" and reasonably priced. I would not get the forged chuck, they are usually for higher RPM turning. They also have a set-tru type which is about 1/2 the price of a Bison or PDA.
KU 202-1791 CS63 6-1/4" 3 Jaw Cast Steel Self Centering Scroll Lathe Chuck TIR 0.0012
http://www.shars.com/products/toolh...-cast-steel-self-centering-scroll-lathe-chuck
SKU 202-5463 6" 3 Jaw .0005" Adjustable Universal Chuck 2 Piece Jaws TIR 0.0005
http://www.shars.com/products/toolh...-0005-adjustable-universal-chuck-2-piece-jaws