I think this is a valid consideration. But in reality, the Ipad does not really back up your photos. It will transfer your photos, but they will be compressed and at reduced resolution. What the ipad can do is to work as a ad-hoc photo viewer and to upload to the internet if u have 3G.
The P5000/6000 probably will back up your photos, but I think the screen being smaller, not so shiok as a viewer.