The size and software differences are minimal, not worth the money. The battery life is longer though, and the grey click-wheel...hmm, well, you love it or you hate it.
If your 3g iPod works fine, and the battery life isn't too much of an issue (just about 2-3hours more), stay with it. The upgrade isn't worth the money...
And if you like the Touchwheel, all-white front and the red light buttons, you will be disappointed with the click-wheel, which is ugly grey. But that's my personal opinion anyway...
