Both products delay the drying time of acrylic paint with a maximum of 20%, the biggest difference being that the slow drying medium is a pre-mixed, ready-to-use medium that also contains acrylic resin, which makes it much easier and safe to use in any quantity. Like with any medium, adding a large amount will make the paint thinner and more transparent.
The acrylic retarder, on the other hand, has little effect on the viscosity and opacity of the paint. You should be careful adding the acrylic retarder to your paint, though, and only add a maximum of 5%. If you add too much, your paint will turn rubber-like and become unusable.