I have a 1D3 and an XTi. My girlfriend has a 40D. Therefore, I get to play with three 2007-vintage Canon 10.1mp cameras on a regular basis. Let me explain my habits this way:


If I need a camera, I grab my 1D3.


If I want a second camera, I hope that my girlfriend isn't using her 40D, and if not, I take it.


Only if I/we have a use for a third camera do I go find the XTi.


The 1D3 is a fantastic camera, incredibly customizable, responsive, and the pictures are great. The Err99 issues are occasionally a pain (lately I have to turn it on and shoot a frame FAST, or I'll get an Err99 every time), but I haven't tried Roger Cicala's thoughts on easy remedies yet. The AF is still a little suspect. Nonetheless, it's the first camera I grab.


Lens selection can be a bit difficult. Most of the lens line is geared for full frame or EF-S. 24-105 is just not that wide, but EF-S lenses won't work instead. As far as size/weight, I just deal. I had a Minolta 7D and I always used it with a grip except when I'd do 150-mile weekend bicycling events. Chances are the larger body will fit just about any bag you might choose, if it has enough room for your lenses.


I don't think built-in flashes can do high-speed sync, so in my eyes I'm not giving up much without a built-in flash. I don't mind transporting the flash, since I'm usually carrying 1-3 other flashes for off-camera work, but I do get a little tired of dealing with a flash when dangling the camera from a shoulder - I'd rather the camera just rest its hot shoe against me than having to find a comfortable point for the flash to bump me.


No experience with the 400 DO, but it's highest on my "most likely to rent for this summer's sailing trip" list, assuming I can afford the 10-day rental.