joezasada wrote:
a used 1D2 will cost the same and be a better choice!
I agree, sort of. From a performance standard and framing (both fast as heck and 1.3 crop) they are the same. But, the control sequences and menus on the 1D2 are pretty different. So that means getting used to the two buttons to do anything of the older 1 series bodies and hunting around to find settings. Also, the Custom Functions set is different. Not huge problems but something to consider.
My Rebel XSi is typically my 1D3 backup, which is fine for wedding/event photography, but if you're shooting something like action sports a 40D would be a better choice due to faster frame rate and higher ISO performance. I wouldn't carry a 1D2 as a backup to my 1D3 simply because I don't want to carry that extra weight around all day. The 50D would be an excellent backup as well but the 40D is a much better value at this point.
I'd go for a 40D. MUCH better value as Chris said. Both are 10mp, similar menus, and compliment each other up to ISO 800 (1DmkIII is cleaner 1600 and up).
Depends what you want a backup for. If it's strickly sports and you can't afford another mkIII, the mkII is a better choice than the 50D. (and the mkIIN is better than the II but 50% more).
Whether you go with a mkII or a 50D the interfaces are different, the CF sets are different and the batteries are different.
For general shooting, the 50D is a competent choice for a backup. I've owned a mkII, mkIIN, 5D, 1D and a 40D. I'm "downgrading from the mkIIN to the 50D as a backup to my mkIII - 1 for the cost differential and 2 because there are times when I want a smaller body to carry around for casual use and the 1 series just can't do that.
I've used the 50D for some fill-in shots with the MKIII as the prime and it was quite nice. Quick, nice handling, and excellent detail and IQ. I think the 50D would work just fine.