Don't get me wrong, I love my 3D printers, lasers and CNC machines, but there comes a point where you need something a bit more robust.
When your robot is going into battle, only metal will do!
In terms of robot competitions, especially in younger brackets, you will usually see more Vex than Actobotics. I had to take into account my use case and availability, so I went with Acto but they are pretty similar and can even be connected in some cases.
For my builds I have used a fair amount of 15-15 (Openbeam) and 20-20. The convenience of these modular pieces is the pre-drilled and spaced holes, the only problem is they use imperial units for some bizarre reason!