I am assuming that by speed you are referring to FPS so I surmise your into shooting sports. If so a upgrade to the 40 will give you a few more frames per second and at times it can make the difference between getting or missing the shot.
Roy Hernandez wrote in post #4154946
Yep! as everybody said... go with the 40D, now is the time to improve your photo quality and learn more new things that the 350D cant do!
The 40D will not improve your photo quality very much at all nor is there anything new to learn. The XT has the came capability as the 40 nearly feature by feature. The 40 has a couple more MPixels but unless you printing very large pics, you'll not see that improvement. The 40 has better ergonomics but that wont improve IQ either. If you need live view, thats it a great reason to upgrade as the XT does not have that.
bauerman wrote in post #4155004
$1000 invested in a lens will usually be better than $1000 invested in a body.
I agree, glass beats body every time and sometimes the difference is astounding. Below is a pic I took a while ago comparing the kit lens to the 17-55. Same body, tripod mounted same settings. I took several shots allowing the lens to focus each time and took the best from each though there really wasn't much difference between the series. Based on your current lenses, I recommend going for the glass. You'll see a huge improvement with the very first pic.
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