A 2003 Vitz should give 18+/litre.
I think Vitz is a better option as u wud get more value for money. Lots of features that r non existent in Santro e.g., Power windows, mirrors, central locking, a reliable toyota engine, power steering, comfortable drive as well as comfortable rear seats etc etc.
Thats just my opinion, decision is urs considering ur own other concerns.