I think the 20D and 30D are obsolete.
There is a major difference between the XT and the XTi.
Stay with an XTi and if you want a better full-frame body save up and get the 5D.
The 20D (discontinued) and 30D and not full-frame either.
The XTi with it's cleaning sensor is more advanced than the 20D and 30D... Canon will be coming out with a newer 30D soon. Put your money into good "L" lens and a XTi --- and then save up for a 5D or a 1D
The 20D is 3504 x 2336 (8.2 MP) 1.3 crop factor
The 30D is 3504 x 2336 (8.2 MP) 1.3 crop factor
The XTi is 3888 x 2592 (10.1 MP) 1.6 crop factor
The 5D is 4368 x 2912 (12 MP) 1.0 full frame
Well, this isn't helping my point, but it's worth noting that that 20D/30D crop factor is 1.6x not 1.3 as far as I know.
That said, there are a bunch of reasons why you might go 30D over an XTi. For starters, if you're into concert photography (low light / no flash), there are two key things the 30D has on the rebel: (1) ISO3200, and (2) a spot metering mode. In addition, the low light capability might actually be worse on the XTi because of the added pixels on the same image size.
Beyond that, you could throw in the fact that it uses a bigger and longer-lasting battery (about twice the capacity), and if you know anything about flash, the 30D's max sync is at 1/250 whereas the XTi's is 1/200, that is a difference that matters when it comes to flash for all sorts of complicated reasons. Not to mention that the 30D has separate controls for exposure compensation and flash compensation, whereas they are combined into one adjustment on the XTi -- another absolutely key feature of a more pro camera for anyone using flash. Basically, with a 30D you can dial down the flash compension for fill flash outdoors without touching exposure compensation, with an XTi the only thing you can do is bring down your exposure compensation, which is worthless in that type of a situation (among other scenarios).
For the fast shooters out there, the 30D can do 5 frames/sec, which is 66% faster than the XTi. And, the max shutter is 1/8000 on the 30D, but only 1/4000 on the XTi. This is not only important for fast action shots, but more importantly, for using larger apertures to blur the background in outdoor lighting or other bright environments.
And lastly, the 30D has a few useability form factor features that are improved over the rebel. First, it is bigger and more solid, it feels like a pro camera, where the XTi feels more like a plastic toy. This may not matter to some, but it'll become seriously obvious the minute you try to slap a 70-200mm lens on it. The 30D also has a sweet extra joystick on the back that can be used to pick your focus point or other things depending on custom fuctions. Seems like a small thing, but it makes a difference. And lastly, the 30D shows a bunch of info inside the viewfinder, things like speed, aperture, exposure compensation, etc. This is very, very nice cause your eye doesn't have to leave the viewfinder to change things up in the heat of battle. Now, I'm not sure maybe the XTi has the joystick and the in-eye display, but the older XT definitely did not.
Anyway, the point is, there are definitely additional features to the 30D, and the 30D is certainly not "obsolete." These features for some are simply not worth it, but for others, the combination of these additional things make the 30D much more of a "pro" camera than the XTi is. They are subtle, but they are not trivial, especially the faster sync speed, flash compensation, spot meter, 3200 ISO, faster shutter, and improved durability and ergonomics.
Topic: Let's talk Canon digital SLRs... (Read 83067 times)