I have a mix of steel and plastic. I use the steel one's for heavy objects (hammers, C-clamps stacked 6-8 deep) or for tools with small holes in the handle that won't fit on the thicker plastic hangers (my utility knives are a good example).
But for everything else, I use plastic whenever I can. Why? Because the metal one's are forever falling out of the pegboard when I remove the tool from the board. You can't (generally) just reach up with one hand and grab the tool you want. You have to stretch across your bench with both hands, grabbing the tool with one and the hook with the other, to stop the hook from falling out. A minor inconvenience, but when your benches are as buried in stuff as mine are, it's can be a PITA to find those hook once they fall down!
The plastic one's have a split bulb end, which makes them stay SOLIDLY in place on the board, allowing for one handed tool removal. I've NEVER had one of these come out. And so far, I've never broken one.
If your not sure, just buy a small quantity and try them. You'll see what I mean.
