That’s really difficult to say, since as far as I know, most brands have different ideas of what “light” and “hard” are. I’ve gotten bands that felt almost like nothing, and some that are incredibly crazy. It all depends, and there is often not much indication on most retailers.
Also, most people do not use resistance bands for stretching, though in certain cases I could see it being done. In those cases, those bands may be “encouraging” you past what you should be doing, or into places where your body cannot resist.
For that reason, I would suggest going for the lightest you can find, and then seeing what it’s like from there. Or, you could get yourself a yoga strap or some other, non-stretchy material to test it out. You may prefer to stretch your muscles without the variable of your band stretching as well.