Hello sleepyboaby.
Just to let you know, in our opinion we think the upgrade is worth it!
We stayed here in summer 2007 with some friends. Four families had premium apartments, and one had a standard apartment as they booked late and the premium ones were booked.
There is quite a difference, in space and decor, especially the ground floor premium rooms which have lovely big balconies that you can go straight out onto the pool area (there is an upgraded pool area too, with bar, for the sole use of the premium apartments).
Our friends room was a lot smaller, and the pool area can be a bit manic with lots of loungers all quite close and noisy entertainment, lots of kids and a busy bar area.
However, in these credit crunch times, I would consider the money as extra spending money whereas my hubsand would say UPGRADE definatley!!!!
All depends how much time you will be in apartments and if you have children. (if you do, try to get ground floor....so much easier)
Must admit kitchen/diner was lovely in premium with marble surfaces etc.
Whatever you decide, sure you will have a lovely holiday there, Duva is very nice with lovely staff.