In a nutshell, throughout the US, ESRI's World Imagery layer provides a resolution of 1m per pixel or better. Bing aerial maps as well offer worldwide orthographic aerial and satellite imagery. Like World Imagery Coverage the quality varies by region, with the currently most detailed coverage in the US and UK.
Both Bing Maps and Word Imagery can be loaded directly from as layers in ArcGIS Desktop. (Access is by subscription only). Bing maps require ArcGIS 9.3.1, while World Imagery works with ArcGIS 9.3. The direct link for the World Imagery layer is here, and here is the one for Bing Aerial Maps.
I had seen a comparison between those two layers, zoomed in to a downtown area in Redwood City, CA. The quality of the Bing Maps by far superseded the World Imagery. It provides stunningly crisp detail compared to the World Imagery, which appeared fuzzy and pixelated. So when I first loaded Bing Maps I was disappointed to find that for our particular study area the quality of the Bing Map turned out to be quite different.
For the area that I had chosen arbitrarily, the World Imagery appeared to be much crisper than Bing Maps. However, when I zoomed in, the Bing Map does show the building clearer than World Imagery and certainly with better colors.
In order to compare the image quality in urban areas I picked a downtown area of Charleston, SC, again for 2 different zoom levels. Clearly World Imagery outperforms Bing Maps on the larger scale. As we zoom in, the quality of the Bing Map improves. Interestingly, though, in this particular image the shot of the building is taken at a slight angle, which makes it appears slightly skewed and the front columns become visible, In the World Imagery the angle of the shot seems to be much closer to a vertical 90 degrees.
Finally I looked at a much more remote area with abandoned rice fields. Here, World Imagery outperforms Bing Maps on both zoom levels.
These results are certainly not surprising. We know that imagery is composed from multiple sources with different levels of quality. What can be seen here is, how this plays out for particular areas as well as zoom levels. At least in the US it appears, Bing Maps prefer urban over rural areas, whereas World Imagery covers both more evenly.