Thanks for your question Jason. We know it can be a tough choice, particularly with the prices between different versions of the iPod getting smaller.
In terms of performance, the 40GB fourth generation iPod and the 30GB iPod photo are practically identical - they both deliver a nice, clean sound, although the bass can be a little lacking at times. When it comes to battery life, the iPod photo pips out the fourth generation model, being rated at 15 hours worth of life (as opposed to 12 hours for the fourth gen).
If capacity is all you're after, then go for the 40GB fourth gen iPod. But as you're going away on a trip, the iPod photo may be a better choice. Assuming that you have a digital camera, the 30GB iPod photo lets you immediately store and view any images (with the addition of the optional iPod Camera Connector for AU$48)). This means you don't have to carry around too many spare memory cards for your camera, as you can hook it up to the iPod and download anything you've already taken. You can, of course, also use the fourth gen iPod as a storage device for your images using devices like Belkin's Camera Link (AU$109.95) or Digital Media Reader (AU$139.95) add-ons. But you won't get the instant gratification of being able to see your photos on a colour screen.
If it was my choice, I'd go for the 30GB iPod Photo, mainly because having a colour screen is just plain cool. But if you're not fussed about digital images and you've got a massive collection of music you need to store, then stick with the fourth gen iPod.