Steel.
I carry and have mostly revolvers.
But the semi's I have owned and still own are steel.
I did try a Ruger P345 for a while. It carried well, and functioned flawlessly.
But I ended up selling it to go back to steel. I tried, for two years, but couldn't get used to the felt flexing / twist in the grip when I fired it. I know it was miniscule, but I could feel it, and after while it bugged me to no end.