Effectively in terms of image quality, it would seem the 300 IS
a 10D wrapped in sheeps clothing, so any 10D/D30 image comparisons are probably a very good indication. And of course you're doubling the pixel count.
In terms of handling - well the big plus is a decent AF
system on the 300D.
Personally I'd have some reservations about pre-programmed exposure and focus modes and the D30 might still be a more flexible tool, but overall, I'd say you'd have a better chance of getting the shot with the 300D.
Good though the D30 was, I think its now well eclipsed in terms of value for money by the 300D - though of course, the great image captured by the D30 2 years ago are just as great today. If you're happy with the D30 and can live with its limitations, you might be better investing in some new glass.
Colin K. Work, Pixstel