Both make good cameras, and both have their fans - tangcla's suggestion of seeing what you can scab off your mates is a good starting point haha
I am definitely not a professional, but I found that the 400D twin lens kit suited me perfectly. The twin lens kit gives you an 18-55mm and a 75-300mm lens IIRC, which are good enough for my purposes, though I'd love to be able to afford some higher-end ones. It's a brilliant bit of gear, particularly for the money - it's certainly capable of taking much better pictures than I am. It'll do long exposures, short exposures down to 1/6400 I think, heaps of expandability (lenses and flashes etc are interchangeable with the higher-end models so there are plenty out there), manual aperture, remote firing, etc etc etc. It even comes with software that you can use to run it with a laptop, which is good for lightning shots for example - better than standing there pressing the button every few seconds. Spend the $150 or so on the 50mm F1.8 prime lens, along with a spare battery and a few filters if you like, and you have a pretty decent entry-level setup. A speedlite makes a huge difference once you learn how to use it too, I personally got the 430EX because I don't need to run a slave flash, and if I ever do, I'll buy a 580EX and use the one I have now as the slave.
By the sounds of it, IMHO I'd say you're spot on with the 450D
