Titanium because the ionic bonds between the titanium molecules are stronger than steels,titanium bar is much stronger than steel bar but much less dense. It is therefore important as an titanium alloying agent with many metals including aluminium, molybdenum and iron.
Titanium bar has better high temperature strength than steel bar. Titanium bar doesn't resist shocks like steel can. It's more brittle. It's also far more difficult to weld. Titanium must be welded in an oxygenless environment. The average strength of steels that are made is under that of titanium. However, the trongest steel is stronger than the strongest titanium. Steel is the only viable building material. Titanium is too rare, expensive, and hard to handle for structures of great height.