I've tried both, but have always been slightly turned off by the idea of drinking polyethylene glycol (MiraLax). Magnesium Glycinate works well for me, but I also have to eat more fiber (salad, beans, etc) and drink plenty of water. And my constipation problems aren't as constant and severe as they have been in the past.
At least the magnesium adds something toward healthy electrolyte levels, and supposedly, most of us tend to be low on mag.
I say, whatever works better for you.
Calli