a used car will always be a used one. Better get a brand new with warranty and parts availability.
Other issue (for me atleast) is the 3 cylinder engine on Vitz which tend to develop vibrations in the longer run. Swift looks more modern and if you go for GLX, you will get a lot more features as well.
On the other hands, both are excellent cars. If you usually carry 3-4 people on board, then Vitz is a better choice since swift's cabin is not as spacious. but if you usually travel alone or with 1 passenger, i would definitely recommend you go for Swift.