I highly prefers 7 days jams over 48 or 72h ones.
In a 7 days jam I can spend 1 or 2 days trying new stuff (graphical styles, gameplay, technologies) and make a good game, original and well polished. This makes for a better learning experience and better end product.
My 2/3 days jams ended up with half playable prototypes, and me not learning much.