Get the 30D over the 400D no question.
That'd be a tough one for me. It was difficult enough for me to choose my 350D over the 30D when I was in the market (before the 400D was released). The 350D and 30D have extremely similar image quality and resolution, and in the end, it's all about the images, right? Sure, the 30D has a few more features, slightly faster write speeds, and is larger overall than the 350D, but it's also quite a bit more expensive. And since the 400D was released, it offers even more reason to go with the Rebel line. The shooting performance is on par (if not better), the image quality is at least as good, the image resolution is higher, it's lighter, it's more compact, and it's less expensive than the 30D. Of course, the 30D has its advantages as well... Magnesium chassis, for example. But the features come at a hefty price premium compared to the 400D, and in the end, the image quality and resolution aren't any better.
Don't get me wrong. I'd highly recommend either of these two cameras over anything else in their respective categories. They're both very capable and high quality. I guess it just depends on each individual's particular needs and wants.