Hi Andy welcome to WPF
We regard to image quaility there is very little to chose between them. It depends on wether you feel is better to spend the extra on better features and build quality or put the money towards a good lens. To many people focus on the camera when they should also research into the lens.
If you are serious about wide angle and wedding work maybe a 5D full frame is a better choice if you can afford it. Both the 350D and 20D are good for long telephoto work because of the high resolution and crop factor. If these cameras were full frame they would be 20mp.
I suggest if you have not already done so compare the 2 cameras in the hand. the 350D is much smaller than the 20D.
I went for the 20D for the following reasons when upgrading from a 300D.
I felt the 350D was too small for my hand especially if large heavy lens are fitted. Mind you some folk have bought the 350D battery grip to get around this problem.
Better build qualitiy. I droped my 20D a few months back. The camera OK but lens in for repair.
I prefered the viewfinder on the 20D
Longer battery life. ( also used the same batteries as 300D so I had some more spares )
A link comparing the two cameras.
http://bobatkins.photo.net/photograp...xt_vs_20d.html
There are a lot of people using the 350D for wildlife so if money is a bit tight for camera and a good lens then go the 350D route. There is no point in having good camera and a cheap lens. I would also buy a body only and buy a better lens than the kit 18-55 lens ( this lens is quite poor, the only thing in its favour is that it is a very cheap wide angle lens )