A used Canon 5D sells for what many new DSLRs cost, but it's full frame.  Other than movie mode do newer, non-full-frame DSLRs improve on the Canon?

The 5D MK II has a very good sensor.  DxO Mark (http://www.dxomark.com/index.php/Cameras/Camera-Sensor-Ratings) ranks it 14th overall out of 148 tested sensors (including medium format) and 5th for sports and low-light performance behind the newest generation of Nikon cameras.  It is ranked higher than Canon's 1Ds MK III which sells for several times the 5D MK II price. When you compare price to performance, it is one of the best cameras available for the money.

The marketing term "Full Frame" is misleading.  The imaging area of a sensor is always a "crop" of the focal plane image because digital camera design has inherited the concept of a rectangular image. So, as long as we have a rectangular aesthetic in camera design, we will always crop some of the image. Marketing folks use the term "full-frame" to describe a camera system that approximates the size of a 35mm negative for use with lenses that have been designed to provide an image circle of similar size. The reason that a 50mm lens appears "normal" on a 35mm body and a 50mm lens appears to be "wide angle" on a 6x6 MF body, is due to the relationship between the focal length and the size of the imaging area. It is the size of the imaging area that is different, and therefore, the coverage required, and therefore, the angle of view. You should be concerned with three key attributes: 1.  The angle of view you desire in your images (the "width") and your existing lens kit. 2.  The tolerance for "aberrations" that you have (image defects). 3.  The typical conditions in which you will be shooting. If you do not currently rely on a 24mm lens to be "wide" and can buy a new wide angle lens, a crop body may be for you.  This leads us to question two: if you do not plan to print or severely post-crop your images, you may not need the extra coverage that a full frame body provides.  Lenses tend to be manufactured for "full frame" coverage regardless of the body used, unless you buy specialty lenses like DX vs. FX in the Nikon system.  As a result, aberrations become perceptually bigger in the final image.  If this is not an issue for you, that brings us to question three.  If you are typically shooting in well-lit conditions, it's very likely that you won't encounter issues with noise, regardless of the type sensor you chose. Finally, there is a relationship between quality at an EI of 100 and an EI of 1600, and it's usually because of the noise reduction algorithms that the camera company has employed for use with a given exposure index and the way the sensor manufacturer has determine the REI - the recommended exposure index. The historical phrase "ISO" is also misleading when discussing digital sensors because it conjures up ideas from film.  For more on this, you can read about ISO 12232:2006.

Can a smaller sensor compare with respect to image quality? Noise: It may, factors other than size matter (such as the microlens, the bayer filter, the size of the photodiode compared to the total pixel area, the amount of noise in the electronics, etc.  You likely do care about this even if you do not shoot at a high 'ISO'-equivalent because it can affect shadow areas at any speed setting. Composition: If you are intent on wide angle, the larger sensor is 'better'. Depending on the pixel count, a smaller sensor may be better for distant subjects (it may also have the same 'resolution' as if you just cropped the image from a full frame) Shooting speed: Hight resolutions generally imply slower frame rates. If you are happy with 4-6FPS you do not care about this.