This really depends on your personal preference and what look you’re trying to achieve for your build. If you want to keep your Glock looking mostly stock without getting super fancy, go for the aluminum version.  If you want the most “high speed” metal, that would be titanium. It’s the lightest and strongest by far. Plus, it’s just cool.  If you’re part of the “Patina, Patina, Patina” crew, go with brass or copper. If you’d rather not wait for the natural patina, you can speed up the process using some vinegar and salt (search YouTube).