Both, plus lightweight alloys. Colt Commander and Lightweight Commander in 9x19, and similar 1911s, are great metal frame 9mms. Likewise the BHP which also been made with an aluminum frame. CZ makes some great poly and metal frame 9mms too. The great S&W 3rd Gen. autos chambered in 9mm were offered in steel, aluminum, and even scandium frames. SIG P series are another great group of metal frame autos offered in 9mm.
There have also been some great plastic frame 9mms from Glock, HK, Springfield / IM Metal, S&W, and others.
I like plastic frame and metal frame semi-autos. You have some very nice plastic frame 9mms, but you should at least shoot a few of the classic metal frame 9mm pistols for the experience.