Actually its neither.
It is true that we used to think that this and many other planetary nebula were "limb-brightened bubbles".
Now it looks like most planetary nebulae are rings (more like a donut than a cylinder).
But even this isn't settled. Using observations of molecules, we can use the Doppler effect and determine how the gas is moving which helps to build a 3-D picture. The Ring Nebula looks like it might be (American) football or Rugby ball shaped and limb brightened.
And this is embedded in something larger and fainter that is more spherical.
Also - if you search for infrared images (e.g. from Spitzer), in molecular hydrogen, the Ring Nebula is much bigger and looks like a flower.
ADDED:
here's the Spitzer Space telescope image:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-07/release.shtml
and here's the Wikipedia entry that talks about shape - it seems to be remarkably up to date (describes it as a prolate spheroid = football)
http://en.wikipedia.org/wiki/Ring_Nebula#Nebula_structure