Mucksavage,
Many good insights, and here's mine after shooting BOTH the Canon 40D (a customer's rig I delivered and trained him on) and my own current lowly little Canon 450D (Canon Rebel XSi)
1) Alcina is right that with rebates the 40D body is very close in price to the NEW 450D. S at $799 for the body or $899 with the NEW 18-55mm IS kit lens (highly recommended!) you are paying top dollar for the newest gizmo out there in the 450D. Both have Canon's DIGIC III generation processor and this makes a great difference in the resulting photos from either camera. With the 450D you DO get 12.2 Megapixels and Canon's legendary CMOS sensor and continually improving with each generation noise suppression algorithms. Read various camera reviews at
Digital Camera Reviews and News: Digital Photography Review: Forums, Glossary, FAQ and
Steve's Digicams - Main Menu and see how well this little rocket works.
2) Live VIEW on either camera will not be that useful underwater. The 40D actually has a bit clunkier Live View button pushing scenario than the 450D. I tried the 450F Live View in a pool last week and while it MIGHT be useful for sessile macro critters I will still shoot through my viewfinder
Get a black silicone low profile mask and practice "seeing" the edges of your viewfinder and use either Canon model's HUGE 3" screen for any menu changes and reviewing your pics. The Ikelite' housing's Super Eye will make either work just fine.
By the way, the 450D viewfinder while still a Penta-Mirror versus the Canon 40D Penta-Prism is much brighter and easier to see than the XTi model preceding it. But hell, I have used them all UW: Canon 5D, 40D, 20D , 30D, Digital Rebel, XT and XTi and made photos with them all.
3) AF mode on 450D has cross type sensors on the center point only, 40D on all sensors. Not a biggie if you use your camera enough and practice AF focussing. I wear glasses and have an Atomic low profile Rx mask and I find either viewfinder adequate.
4) Having a second "Command Dial" rear control on 40D you'd think is Nirvana according to some UW shooters over ONE Command Dial as the 450D has. Holding in a little button with your thumb (surface use) or the "press in and it holds" control Ikelite implemented from the first original Canon Digital Rebel mode works fine.
Good underwater photography is more about setting up a shot and then minimizing parameters to adjust when that grouper yawns, shark swims by, angelfish is teasing you in and out of a hole, etc.
I shoot "M" manual mode when paired with Ikelite's TTL system and DS 125 strobes that STILL give me TTL fill in flash much faster and easier than any manually adjusting mumbo jumbo......
5) Camera weight.......I'm lazy and after traveling a LOT and usually wanting to have two camera bodies eventually (one in the housing, one for surface shooting and back up) I purposely chose the 450D. And this was after going back and forth in my mind for several months. I LIKE the lighter weight meaning I will take my camera places I might leave a heavier slightly more bulky rig behind.....
6) Build quality is something I read about over and over as reasons for a heavier, sometimes more costly model. Yet after owning the original Digital Rebel, TWO Rebel XT and XTi bodies with over 20,000 clicks on each in a year and now the XSi (450D) I have never wanted for any more ruggedness. The 450D even has a bit more solid feel to it than previous "consumer" models. My recommendation is to spend more $$$ on lenses! That said, the 40D is a great camera......
Either model you can't go wrong. Just don't buy something because people will think it looks or is touted as more "professional". What hogwash.........The only thing that counts is your photos. I se a lot of "snapshots' taken with thousands of dollars of equipment.
And I also see people shooting and selling work with a Canon G9, and other lower priced cameras every day.
Hope I didn't confuse you
dhaas
David Haas Underwater Photography