Compare my rebel Xti to the eos 6d.

[Skirball]

  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.

[hsbn]

		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

From here: http://snapsort.com/compare/Canon-EOS-6D-vs-Canon_EOS_Digital_Rebel_XTi

It's a great site to compare camera spec.