Trial & error is how I chose our curriculum. You have to look at your teaching style and the child's learning style, and also your teaching philosophy. You have to look at cost as well. To me, it's better to spend a little more on something that can be reused for siblings and then sold than to spend a little less on something that will have to be replaced for child #2 and can't be sold later.
Mostly, just ask questions here (what are good math programs for a child who ____), and then research those suggestions with something like this:
http://www.cathyduffyreviews.com/.