It certainly won't be before 3rd April, as the current offers run until 2nd April, and then SEAT will only change it if the car isn't selling (which it is).
Currently, the Ibiiza gets £1,500 at 6.1% APR whereas the Arona gets £1,000 at 4.9% APR. Add to that the much better residual values on the Arona and, as I proved to a customer this morning, a more expensive Arona (despite "only" having a £1,000 deposit contribution) can actually work out cheaper per month.