The 6D and 5D III are goin to perform extremely well in low light. If you're on a tripod, any camera will perform well in low-light (which opens up the options).
The 6D wlll be more budget friendly -- it's the "entry" level full-frame body. The 5D III is a more advanced/technical camera. There's also the 5Ds and 5Dsr... the 5Dsr will be better for "landscapes" and the 5Ds will be better for "architecture". But the 5Ds and 5Dsr are 50 megapixel cameras and don't have the low-light performance of the 5D III or 6D. But due to the very high resolution sensor, producing images suitable for very large prints is absolutely not a problem.
A 6D has a 20 megapixel sensor with 5472 x 3648. If you were printing a 16x24 that would be 228 dpi print resolution (300 dpi is somewhat ideal and anything less than 100 dpi is probably unacceptably low unless it's being viewed from a distance.)
A 5D III has a 22.3 megapixel sensor with 5760 x 3840. At 16 x 24 that would be 240 dpi.
But a 5Ds is a 50.6 megapixel sensor with 8688 x 5792. At 16 x 24 that would be 362 dpi. Even a 20 x 30 would be 289 dpi. a 24 x 36" print would be 241 dpi!
The difference between the 5Ds and 5Dsr is the presence of the low-pass filter. When you take images that have patterns of lines and record it on a sensor which has pixels in neat rows and columns, the result would normally create a moiré pattern -- which would probably ruin your image. So most cameras have a "low pass" pass filter in front of the sensor which provides some anti-aliasing (softening) that eliminates (or at least substantially reduces) the moiré. But the trade-off to the low-pass filter is that slight "softening" effect. Without the low-pass filter, images would technically be just a tiny bit sharper.
When you shoot natural subjects such as landscapes, there's not much that could create a moiré pattern. So in those situations you could actually get a sharper image by using a camera that doesn't have that filter (that's the 5Dsr).
But when you shoot "architecture", buildings have straight lines, bricks have patterns, etc. and it's a high probability that you'll end up with that undesirable moiré -- hence the 5Ds (which does include the low-pass filter) is the better choice.
Your Rebel XT is an 8 megapixel camera. On a 16 x 24 it would only have 144 dpi. That's usable as long as you don't look at the images too closely.
Tim Campbell
5D III, 5D IV, 60Da
