It's a new world out there.
I shot an EOS 1N and an A2 backup, each mated to L series glass, and tried to make a go as a motorsports photographer. Got my work published, got to see Indycars for free for six and a half years, and loved it.
Toward the end I ran out of time and money and had to let go...so I sold the 1N. I kept the A2, and used a Rebel (yes, a Rebel. Not a rebel XT...a $90 used 35mm Rebel) as my backup.
The A2 ran 6FPS easy, but besides that the only difference was the Rebel was better at spot metering. I never missed my 1N, except it was massive and easy to hold on to, where the Rebel was a toy.
Not so much anymore, huh? When the bodies just transported film, this was a non-issue except for people who manhandled their cameras (mine have a lot of battle scars on them). Oddly, my little Rebel sustained very little battle damage, maybe bacause it bounced off things, it was so light...
The xsi is what, 12MP? My 40D is 10.1. The web site states both cameras use DCF2.0, 14 bit original images with the nod going to the xsi for capture denisty, and it's not looking good for those of wannabe amateurs like me who plunked down a lot of extra Ducats for the 40D in order to get a tougher body with a lots of throughput and a few cute features.