On my folding bike with a 33.9mm seat post, The quick release clamp has become really hard to operate because the brass bushing and the butt of the lever have worn down after 7 years of use. The brass bushing is a really thin plate that's easy to deform.

There's one OEM model replacement widely available but it comes in varied packages, some with a similar thin brass bushing or with a thick plastic POM.

Would the brass bushing or the plastic POM perform better, be more stable under the high bending of the really long folding bike seat post, or last longer?

I assume the POM is easier and smoother to operate, but some risk of it falling apart if clamped tightly.

Buy one of each, then pick one to install until either you decide it's not working for you or it fails entirely. Then install a different one.

Well, I don't have space to store unused parts since I have to carry everything with me. So I can only choose one.

Plastic can age and become brittle, and also "creep" over time. I'd go with one with a brass or copper sheet bushing like originally, BUT, add a touch of lube at that sliding interface. I use a dab of anti-seize, but grease will work fine too. This will make the lever much easier to close, will not cause it to pop open, and improve durability.

If the aluminum QR cam is not too worn, you can also cut a new copper sheet to size after drilling a hole. No copper sheet? Find a penny old enough to be all copper and not copper clad over zinc. Hammer thinner on an anvil or some vises have an anvil area on top back. Might need to anneal the penny with a torch to make the copper more ductile. If you have an empty rifle or pistol cartridge case that is typical brass, that might also provide raw material. I've used a copper-jacketed rifle bullet, melted the lead out, and used the jacket to make a "socket" for a rim with crack at the spoke hole, to transfer spoke load to inner rim wall, just like double socketed rims. That copper jacket would also work for your application.

Polyoxymethylene AKA acetal AKA Delrin is a pretty tough polymer, and it has self-lubricating properties - it's a good material for jockey wheels, but Simplex made whole derailleurs from the stuff.😩 Is the other type brass or bronze? Generally bronze is self lubricating but brass is not. The seatpost should be a good fit in the frame so the clamp is only stopping it from sliding, not seeing much bending force.

^^ (above) Ah, that plastic is acetal or delrin, yeah, I would not hesitate to use that plastic, it's one of the tougher ones, they make ski bindings out of the stuff. But I also like the design with aluminum separated by a thin copper sheet and a touch of lube. It works, and has a graceful and slow degradation/failure mode.

I don't know about bronze being self-lubricating, but there are tons of applications of "DU" bushings, which are metal backing, porous bronze layer, and then coated with PTFE that impregnates down into the porous bronze, very common and durable if not contaminated with dirt and such. A piece of one of those cylindrical bushings in place of the original copper sheet, would be superior, and not require lube.