I've not seen a great difference between the Multigrade and PQ myself, but your mileage may vary. The Cooltone however does produce noticably cooler tones. I keep prints of the same negative using different developers and papers in a binder so that I can do a side-by-side comparison.
I am using the Multigrade and Cooltone developers primarily right now and I find that you can get good control of toning as well as untoned prints by using the different developers. For example, Cooltone dev on Warmtone paper will tone down the effects of Sepia toning, while Multigrade on the same paper allows for a stronger sepia color. The same goes for Selenium toning. The Cooltone developer appears to retard color changes with Selenium, which may or may not be desireable depending on the print.
These are fairly inexpensive (except for the Cooltone and Warmtone developers) so try them out with quarter sheet prints and compare the results. I like to keep multiple developers on hand for greater control of the end product.
In addition to just one developer, there you can bleach back and redevelop in Cooltone developer to get still different results. Try MGWT developed in Multigrade or Warmtone developer, then dichromate bleach the print and redevelop it in Cooltone developer for even cooler blacks.
- Randy