The Tiida is being discontinued, even Nissan admit to the car being a huge flop. It may be cheap but that's for a good reason, you cannot compare it with a Polo, as the Polo is in a different league.
Our Polo 77TSI has been terrific, economical, comfiortable, reasonably quick and has a lot of character.
It's a no brainer between the two, remember you get what you pay for.