Weaker lenses won't show some of the details we need to see but stronger ones will have such shallow depths of field that most of what is looked at will be out of focus. Less powerful lollipop-type magnifying glasses display larger areas than hand lenses and thus are useful for looking at spiders and other small critters you don't want to get too close to. Or maybe you're watching a large number of ants devour a caterpillar, and you want to magnify the whole field of carnage. So, there are uses for the reading kind of magnifying glass, but if you have to choose having just one, the little hand lens is much better. Get lenses that don't scratch easily.
Plastic lenses scratch too much, and uncoated glass lenses scratch more than coated ones.
Try to find one where nearly the whole field is in focus when you look at the whorls on the tip of your finger.