Xxl sounds like a large online manual. At least that's what I got from the home page. Quick load and grt are simulators that let you vary parameters to estimate what the load will do in your chamber/rifle. I used and still have quick load. However I got close models from grt so I have gravitated to that.
I have QL and Gordons. I seem to be using Gordons more and more lately. I find new features all the time and it just gets better. Using all the features in Gordons it has predicted load performance almost exactly. I guess its like anything else only as good as what you put in.