Question: Microsoft sells "full" (non-Arcade) games on its Xbox Live Marketplace as digital downloads ("Games on Demand"). On average, are GoD loading times significantly faster than that of the disc version "installed" to the hard drive? Or are the access times identical because the data and retrieval techniques are identical?

Answer: 
Depends on the game.

If the game has an installer (ala Halo Reach), it will load significantly faster via disc than via the GoD version.

But for the most part, the Games on Demand version will run a lot faster during loads (but some of the loads are specifically hard coded times, such as Mass Effect elevators, so that won't ever get faster).

If you're trying to decide between as disc and GoD, you'll get a better price on a disc, but GoD will probably load faster due to the constant read speeds off a single drive instead of transferring data to RAM, reading from the disc and the HD.
I really can't tell if on average GoD downloads load faster as it seems to vary in a case by case basis. Take Halo 3 for example: back in 2010 it was [acknowledged by Bungie](http://www.1up.com/news/microsoft-acknowledges-longer-load-times) that the GoD version of the game would have longer load times.