It has to be the Touch, doesn't it? I'd say flat-out right now go and buy it, and curse me all you want if you don't feel you've gotten full value for money. The pricing of the Nano is an insult when you compare the features. I was going to get my dad one for his birthday, assuming they were proportionately cheaper, and would've spat out my coffee, if I'd been drinking any, when I saw the prices.
If you don't have an electronic dictionary, you save yourself at least £100.
You can type Japanese (including writing kanji with your finger), which you can save or e-mail.
You can use flash cards (including Anki) wherever you are.
You can read the manga from the store and the e-books from i文庫.
You can view the PDFs.
You can play games like KanjiPop and 四字四字ロジック.
You can watch videos on a screen big enough to read subtitles.
Assuming you have WiFi access, you can go to whatever website you want (including Veoh), e-mail your friends, and well, use the Internet.
You prepare yourself for whatever awesome apps will be released in future
The screen and the amazing multi-touch interface (which you'll definitely love) would justify the price difference for me. Just doing simple things like browsing your music is so much easier. It's also a very sleek, good-looking gadget, and thin enough not to bulge out in your pocket, despite the nice big screen