For your average mixing task, pretty much any bowl (of the right size, shape) will do. Also, "plastic" isn't one material, it's a bunch of different materials. For example, some advantages of polypropylene bowls do not apply to melamine formaldehyde bowls, but both are plastic.
So, here are some advantages of a polypropylene (PP) bowl over a metal bowl:
- microwave safe
- high elasticity (can be bent, and will go back to its original shape. E.g., if you drop your PP bowl, it won't dent)
- extremely acid-resistant
- extremely resistant to other chemical attacks (e.g., will not corrode)
- plastic bowls are often made in a mold, so can be cheaply made in a variety of shapes. You can get bowls with nice pouring spouts, etc. because of this.
Of course, they have disadvantages too:
- fats stick, almost impossible to remove all traces (a problem when beating egg whites)
- can't be used over a pot as a double-boiler
- can't hold very hot things (e.g., you can put hot oil in a metal bowl, but PP would melt. Different plastics have different melting points, some can't even hold boiling water. You never have to worry with stainless.)
- can't be used in the oven
And there are things which are just different, like metal conducts heat better (so it works better when cooling in an ice bath, but also you're much more likely to get burnt on it), metal is denser (PP floats, metal doesn't).
If you get metal bowls, except for a few applications where copper is nice (those egg whites, again), you want stainless steel. Stainless is fairly resistant to acid and also corrosion (plain steel is not). Note that stainless comes in different grades, which vary in their corrosion, rust, and acid resistance.
Best Answer
Maybe! There's one big potential issue: panini presses are hinged differently. There's a bit of variation exactly how, but the effect is that the entire top can lift up, so that it can press down flat on whatever you stick in there. Waffle irons are just designed to be filled, so they hinge at the back. If your sandwich is much smaller than the waffle iron, you can put it in the front part (as far from the hinge as possible), and it won't matter too much, but if it takes up a decent fraction of the area, it'll be squeezed pretty tight at the back compared to the front. This will probably make it cook unevenly, and depending on what you've got in there, might squeeze things out.
Some panini presses also have better temperature controls than waffle irons, but that's probably not as much of a concern.
So I'd give it a shot and see how it works, plan on possibly flipping your sandwich around halfway through, and don't be too upset if it's not pretty!
(rfusca's comment above is also good - if you're able to take off the hinge, you'd avoid the whole problem.)