Plastic. Unless you like lugging around more weight.

I've never had an issue with my plastic ramps. Just do check their weight capacity and make sure your vehicle doesn't exceed that. 

Metal only.  Would you ever buy a rubber crutch?

I would never use ramps anyway.  I use a jack and sturdy metal jack stands.

Do what I did - get some hardwood in good condition and chop up into massive ramps. Mine are about 16" wide, 8" deep and about 4-5" or so long.

Actually I wouldn't recommend that unless you're a carpenter and know what to look out for. But I still feel better crawling under these than whatever garbage ramps they're sticking together in China.

The metal ones are probably made out of tin cans recycled from the west and the plastic out of...well this is a family forum, but use your imagination.

Or maybe go to a metal fabricators and get them to make some if you don't mind the weight?. I used to trust big companies with quality of work, but you just can't bet your life on it these days. But then you've got the local village idiot in the metal fabricators, so all swings and roundabouts.

I bought Blitz Rhino ramps- the heavy duty ones that can support trucks and SUVs.

They're rated for more weight than car ramps but the main reason I got the stronger ones is they give me higher ground clearance making it easier to get under the car with a creeper.

https://tundraheadquarters.com/blitz-rhino-ramps-make-oil-changes-easier/

It looks like they don't make this model any more.  I would not buy the newer ramp made by this company because of this:

https://www.youtube.com/watch?v=GziyxVPI2N8

It takes this guy 2 minutes to get to the point but you can see why you might want to shop for a better ramp.

Here is the previous model available on ebay:

https://www.ebay.com/itm/304525826619?hash=item46e7275a3b:g:hHwAAOSw6z1ilOh~