That’s a very broad question, you’re going to have to be a bit more specific if you want to gain anything useful out of this. But I upgraded from an XTi to a 6D, so I suppose I’ll weigh in a bit:
One is full frame, one is crop. That will affect high ISO performance, field of view, and depth of field, to name a few. It also means that your lenses from the XTi won’t necessarily fit on the 6D. In order to take full advantage of the better sensor on the 6D you need to invest in decent glass.
The “crop factor” is a legitimate aspect of a camera, and it’s both good and bad. You’ll notice a significant reduction in reach moving to a 6D, and getting that reach back can be very expensive.
There’s absolutely no comparison of low light performance between the two. Different league. I also found the color to be noticeable better on the 6D.
The 6D is physically bigger and heavier than the XTi. Still smaller than most full frame and professional grade cameras, but you’ll feel the difference. I like it, but you notice when you travel. The lenses you need are equally bigger and heavier than the Rebel kit lenses.
Upgraded features like custom functions, improved bracketing, WiFi, GPS, HDR, etc. Some may be worth something to you, some may not. I found that I really like Auto ISO in certain situations, I would never use Auto ISO on my XTI. In fact, I found that I now use ISO like I do aperture or shutter speed. On my XTi I would minimize at all cost, on my 6D I have no issues at all going to 1600, and beyond in certain situations.
On the other hand, the 6D is heavier, more expensive, has less lens choices, less reach, less portable, requires more disk space, I worry about it more, is more noticeable on the street, etc. There are times that I take a landscapes in good light or a travel pic or something and think “meh, my XTI could have done that”. There are times I think “a point and shoot could have done that, why am I lugging around a big ole camera”. There’s pros and cons to all camera choices, they’re just tools.
|Much lower noise at high ISO||2,340 ISO||vs||664 ISO||The 6D has excellent image quality 1.8 f-stops higher ISO than the Rebel XTi|
|Much better image quality||82.0||vs||62.0||More than 30% better image quality|
|Much higher resolution screen||1,040k dots||vs||230k dots||More than 4.5x higher resolution screen|
|Much larger screen||3.0"||vs||2.5"||More than 20% larger screen|
|Has live view||Yes||vs||No||Preview your photos on the camera's screen before taking them|
|More dynamic range||12.1 EV||vs||11 EV||1.1 f-stops more dynamic range|
|Shoots movies||1080p @ 30fps||vs||None||Shoots Full HD (1080p) video as well as still pictures|
|Better color depth||23.8 bits||vs||22.1 bits||Distinguishes 1.7 more bits of color|
|Much larger viewfinder||0.71x||vs||0.49x||More than 2x larger viewfinder|
|Faster autofocus||Phase detection||vs||Contrast detection||Catch candid moments easily|
|Much less startup delay||500 ms||vs||2500 ms||5x less delay when turning on|
|Much higher true resolution||20.9 MP||vs||10.1 MP||Capture more than 2x more detail in your photos|
|Has in-camera HDR||Yes||vs||No||Combines multiple exposures to capture high dynamic range|
|Much larger sensor||Full frame 36.0x24.0mm||vs||APS-C 22.2x14.8mm||More than 2.5x larger sensor|
|Weather sealed||Yes||vs||No||Sealed to shoot in the rain|
|Better maximum light sensitivity||25,600 ISO||vs||1,600 ISO||The 6D's maximum light sensitivity is 4 f-stops better|
|Video autofocus||Contrast detection||vs||None||Automatically focuses shooting video|
|Significantly longer battery life||980 shots||vs||370 shots||More than 2.5x more shots per battery charge|
|Has a GPS||Yes||vs||No||Automatically geotags your photos when traveling|
|Has boost ISO||102,400 ISO||vs||None||Allows taking photos in darker situations, often at the cost of additional noise in the image|
|Has a pentaprism viewfinder||Pentaprism||vs||Pentamirror||Pentaprism viewfinders are typically brighter|
|Shoots faster||4.5 fps||vs||3 fps||50% faster continuous shooting|
|Better viewfinder coverage||97%||vs||95%|
Almost the same
It's a great site to compare camera spec.