Whichever way they bend or wear down or whatever really won't matter because speakers vibrate so much they just keep working their way down until they're good and solid and stable. It can take a while, but it happens. That's not your problem. Your problem is to figure out what will sound the best. Which is something no one here can possibly say. But you can figure it out easily enough for yourself.
The easiest/fastest way of doing this, get yourself some wood, some MDF, acrylic, plastic, sorbothane, whatever. Cut these into circles or squares a couple inches across. Circles work better than squares, but this is another one you can try and hear for yourself. Thickness matters too. A good approach would be compare the materials all in whatever thickness you have. Then when you've found the best maybe fine tune by comparing 1/4" MDF with 1/2" MDF, for example.
If you do this I can just about guarantee you will hear differences between everything you try, that each one will have its strength and its weakness, and even that you'll be able to combine them in layers or by using say two MDF and two acrylic under each speaker to have things average out closer to what you want.
That's what I did. Even bothered to compare the little threaded screw that holds the spikes on the speaker. Stainless sounds better than brass or plain steel. To my ears. Yours, who knows? Which is why no matter what it always comes down to the same thing: go and listen.
