PSE3 full or PSE3 upgrade ?

I have got Photoshop Elements 2 two times.
First time with my Canon 20D, second time with my Sony Notebook.

Now I'm dreaming of Photoshop Elements 3

I had a look to the internet shops:

• Approximately 70 Euros for Photoshop Elements 3 (full package)

• Approximately 55 Euros for Photoshop Elements 3 (upgrade package)

Where is the difference?

If I purchase the upgrade, do I have to register via internet or does this upgrade package only look, if there is an older version on the harddisk?

In case I would sell the 20D and the Sony Notebook, could I still use the upgrade package? (ok, doesn't matter so much, probably the 20D successor will come with a newer Elements version).

Further I read that there is a time limit for the upgrade. Is this time limit only related to purchasing in shops or related to installing for the first time?

Now I don't know what to do. 15 Euro price difference is not much.

But anyway, If you intend to buy the 20D successor in future (maybe with Elements 3 or newer), what is the best?

Thank you in advance.