The most notable difference in terms of quality that I have personally noticed is that the geocoding on the OSM side of things is of wildly variable. The Cloudmade v1 Geocoding API was simpler and returned better results that the Cloudmade v2 API, but the data returned in v1 had less detail that the data returned in v2.
Mapquest offer a licensed data geocode and an open data geocode. The licensed data geocode, as you would expect, returns better results than the open data geocode but still fell short of both the ease of use and accuracy of the v1 Cloudmade geocode.
The kicker in all of this is that Cloudmade recently changed the way they choose to do business. That meant that my internal application wouldn't generate enough hits to the the Cloudmade servers to continue to keep the account open. We were literally in a situation of wanting to throw a small chunk of money at Cloudmade but because we couldn't meet a seemingly arbitrary lower limit we couldn't, abandoned Cloudmade and moved everything across to Mapquest.
Also I'd just like to call out that Leaflet.js is a fantastic library to use and allows us to do everything we want to do to the maps with an absolute minimum of fuss. We are looking at adding internal floor plans to some of the maps and I managed to throw together a PoC in leaflet.js that is looking like it would easily support this with a minimum of fuss.