Hi,
I've got a 15GB iPod, and was going to replace it with a 4GB nano but then decided at the last minute to go with the 20GB iPod photo instead.
I am very happy with my decision, which wasn't easy because the nano has massive design appeal.
However, the 20GB iPod photo is only 70 dollars more expensive that the 4GB nano but gives you an extra 16GB.
Also, loads of customers are complaining that the nano scratches too easily. You should absolutely read
this thread and do some research about scratched up and cracked screens before buying a nano.
Sure, people are rationalising having a 4GB nano by saying "I don't need to carry my entire music library around with me, I just make up playlists".
Of course, this is a sacrifice, and it is only a love of the nano's slim design talking.
Nobody would choose to piss around making up favourites' playlists when you can just update your whole library through firewire without having to think about it.
By the way, the nano uses UBS 2.0 instead of Apple's much-vaunted firewire connection.
Personally, I have spontaneous urges for certain songs in my 7 GB music library. They are not guarenteed to be on hand with the nano. Plus I have several audiobooks, one of which alone would take up half the 4GB nano. And breaking up Jim Norton's Naxos audio recording of Ulysses is a prospect up with which I will not put.
So as another poster said, it all comes down to what you need out of an iPod (while factoring in the now quantifiable nano design flaw aforementioned).
You said you will be using an iPod as an external harddisk. If so, the 15GB iPod is the obvious choice.
Hope this helps.