I don't think it's as simple as that. Depending on your needs:

Titanium offers exceptional resistance to permanent deformation; it flexes considerably before taking a set.

Given identical dimensions, steel is far more rigid. Titanium, by contrast, is more elastic and spring-like.

Many heat-treated steels can exceed titanium’s hardness, providing better scratch and wear resistance.

(How many canteens are being heat-treated?)

Titanium is about 45% lighter than steel for a given volume.

This is where titanium truly excels—it delivers maximum tensile strength for minimal weight.

Titanium’s poor heat conductivity makes it behave almost like a thermal insulator—a reason it’s challenging to machine.

Source

I could be wrong, but it sounds like a titanium canteen would accrue damage and dents more easily, but be more likely to survive larger falls. But I'd probably still choose steel just because steel is plenty fine and cheaper.