Tug, I bought these because I already have quite a few identical ones I've had for nearly ten years, here's one of my old ones that's had a lot of use, yes the plastic tips come off after a while, sometimes you get glue under them etc. but it doesn't detract from their use, I've used them on all sorts including Balsa and haven't found they mark the work piece the spring strength is difficult to describe I'd say ample for the sort of joints we encounter, and strong enough to hold my "Helping Hands" aloft, I hope the photo's answer your questions, the middle photo is the jaws being held open as wide as they can.
Joe