All metal construction is definitely better than plastic. But, it is a bit more difficult to find, and of course more expensive when you do find it.
I have 2, the same brand, one is a 25 year old all metal Bernina that was my mother's. The other is a 20 year old metal/plastic Bernina sport (Bernette) that was my first machine. The Bernette served me very well for years, but as soon as I pulled out my mom's old machine and started using it, I really noticed a huge difference. I don't have to worry about the all-metal machine walking or shaking from the vibrations, which means I can sew faster. I only ever use the straight and zig-zag stitches, which means either machine would work just fine, but in use the all-metal machine makes a huge difference in comfort.