There are so many situations where you might use a clamp, so many things you could clamp, would clamp should clamp... Don't sweat it. I use both, and anything else that might work.

Advantages of the steel ones: They have more clamping power, and since they are thinner they fit places the plastic ones don't, and since they're more pincher-like they have concentrated clamping area. Glue seems to release from the rubbery stuff easier than the plastic ones, and if you need really concentrated clamping power in a small area, you can take off the rubbery tip and have just metal.

Advantages of the plastic ones: They're easier to open, I guess, because they don't have as much clamping power, they've got a roundish section between the handles and the tip, so they can hold round things, and the plastic swivelly things are advantageous in some circumstances. Of course, sometimes you want more grippiness, so on a handful of mine I've used contact cement to glue 60-grit sandpaper to the plastic swivelly things.

Don't limit yourself. Just get lots of clamps. There is no clamp that you won't use if you've got it. I used to buy a few clamps every time I went to a box store. Usually just the cheap ones, whatever was on sale. You can't go wrong with more clamps.

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