Tefal is a ceramic based coating (you find it on high-end cookware). So the element is basically a ceramic coated Incoloy metal. Incoloy is a corrosion resistant metal alloy made mostly of iron, nickel, cobalt, chromium and small amount of copper with some trace elements in it. It's probably the Incoloy-DS alloy as that is used in heat-cycle applications. Titanium heaters are as they sound, made of titanium metal.
Titanium heater exchangers are found in high-end (read $$$$) gas pool heaters. Titanium is a very stable metal in water that forms a strong passive oxide layer that can withstand pretty corrosive environments. There's almost no chance of the heater element galvanically corroding.
Ceramic coated metals are fairly inert as well but they can suffer from thermal stress-induced cracking of the ceramic layer. Once the ceramic film is compromised, the Incoloy is exposed to the water environment. Incoloy should be able to withstand spa water conditions but it will not perform as well as titanium.
If I had the choice and the money, I go for the upgrade option and install a titanium element.
