Having used both Marketplace AMIs and non-marketplace (community) AMIs, I find the Marketplace ones to be quite painful to use. I want my tools to be able to go out and get the latest AMI automatically and launch that instance, but it is basically impossible to do this with things in the Marketplace. Amazon makes you jump through a bunch of hoops as if you are paying extra for this marketplace AMI even if it is provided for free.
The nicest experience I've had is with the Ubuntu community AMIs. Ubuntu keeps a listing of their official AMIs[1] and there is a JSON api that they provide so you can fetch that information programtically.