What's the difference in photo quality between pictures taken with a small sensor camera vs a larger sensor?

By small sensor, I mean that of a a point-n-shoot, by large I mean that of an SLR.

Also, if the one has a 10MP point-n-shoot, and a 10MP SLR (such as a Rebel XTi, which is what I have), what's the difference?

Thanks!!!

There are two major differences. The larger sensor will tend to give images with less "noise" in them, particularly in low light.

Also, all else being equal, the larger sensor will have less depth of focus than the smaller one. This means that objects in the background will be more blurred. This is good for some photos, and bad for others.

To stuff 10mp in the small sensor of a point & shoot digicam requires that the pixels be smaller - which makes them less light sensitive.

The following is from "High-Resolution Digicams: Do Major Megapixel Counts Aid Or Hinder Image Quality?" by Peter K. Burian in the Nov. 2007 issue of Shutterbug Magazine:

"As the pixels become smaller they are less sensitive to light (photons). Consequently, "analog gain" must be applied at higher amplification to boost the weaker sensor signal. That produces a lower Signal-to-Noise (S/N) ratio, causing more obvious digital noise - mottled colored specks and/or graininess - particularly at ISO 400 and above."
You can read the entire article at shutterbug.com - just type "the pros and cons of pixel packing" in the Search box.

A 6mp XTi would give superior image quality to a 10mp digicam at any ISO level but especially at higher ISO levels.

There may be no real difference at ISO's like 80 and 100.

As soon as you get past ISO200, the compact camera will show objectionable noise in underexposed areas and in shadows.

By ISO800 the images may be unusable.

The larger sensor camera will produce publishable images up to ISO1600. Top-end cameras will make clean pics at 3200.

HTH
V