I believe that the 350D has the same sensor as the 20D which has improved noise performance over the 300D. Given that the 300D has a formidable reputation when it comes to astrophotography the 350D should be even better than the 300D. From what I've seen there are people getting very good (astro) results from both. The screen size hasn't got much relevance any size on an SLR is too small to be of much use. The 350D has an 8Mp sensor (the Nikon is still 6) which all reviewers agree is a bonus even if only giving a larger image to allow for cropping.
