This question isn't really as straightforward as it sounds. There are really three parameters that determine the suitability of a framing material:
- Strength
- Weight
- Ease of use
Our team has used both Al square stock and extrusion so I can only speek for these two options.
Square stock
Square stock is relatively light and more than rigid enough for a first bot. If you are one of those supposed teams that designs a robot entirely and then assembles it in the last week of the build and it all works properly the first time then this might be a great choice. For the rest of us who are constantly changing things squarestock can become a problem. It very hard to move things that are welded on. If you are looking for strength then this might be the best option. If you are looking for flexibility then look somewere else.
Extrusion
If money is not an issue then this stuff is great. It is probably one of the best thigns our team has ever done. If you need to move something just loosen a bolt. Its very convenient. If flexibility is your goal then this would be the way to go. A word of warning: extrusion has very little torsional rigidity. It can take other loads fine.