Amazing...
Dude takes the time to post a link with formulas confirming that for a given diameter (which is the case in our trucks) the bending resistance will be greater for a solid rod than for a hollow tube of the same material (by a ratio of 85%, it demonstrates), yet people will still recommend to skate hollows to avoid bending...
As for material, a quick search indicates that there is no definitive answer here since we are possibly talking about a great number of different alloys.
The strongest alloy steels are stronger than the strongest titanium alloys (each taken in their hardest tempers), but it's obviously not what they sell you to put under your deck.
Also, the most commonly used alloy of Titanium (6al4v) is significantly stronger than the most commonly used steels.