is alluminium or brass better for steam engine pistons

[Zeb]

If you have a ring groove or two for teflon, the material shouldn't really matter. A design consideration though is the inertial moment of the piston moving back and forth will limit your top speed and create more pronounced vibration. The lighter and thinner, the better.

 

[Steamchick]

Of course, primary mass imbalance can and should be counterbalanced correctly, but secondary imbalances usually need a second balance shaft. And without the second balance shaft secondary imbalance will limit speed during to excessive vibration, very likely before you get to limiting piston speeds on models? But all this needs to be calculated per model so you really know what is best. Some of my slow steam engines are adequately balanced by judicious positioning of out of ballance cast flywheels. Means they run much slower as well. Good for displays to show off the motion.
K2

 

[TSutrina]

​	coef Expansion​	friction Coef​
material​	in/ (in*F)​	on steel​
6000 series Aluminum​	13.1x10-6​	0.47​
Brass​	11.8x10-6​	0.44​
carbon steel hardening grade​	8.3x10-6​	​

Your choice is my opinion.
Use of plastic is far more complex. Plastic absorbs water and expands. Plastic gets weaker as they get hotter. Coefficient of Expansion is higher than metals. Friction coefficient is not give for high temperatures and often the speed of travel is limited due to heating.