Swap 2013 1.4 SEAT ibiza for 08 Fiesta ST?

I'm an 18 year old, and I've had a 1.4 litre SEAT ibiza from brand new, I've had it a year now, so as a part exchange i could get a Ford fiesta ST (08-09) model, what should i do? My car has only done 4000 miles, is it worth changing it? Or keep my car?

And why would you want to do this?