both!
you can probably get good deals on good domains right now, look at dollars.com, sold for 200k higher two years ago.
and you should always be spending a little time on developing, meaning at least building a small information site with affiliate programs and ads to get away from PPC.
when you think about it you just need one domain and one good idea to make a living, look at Facebook and Plenty of Fish, American Flags, Unicycle, etc. Most people don't go that route because it takes a lot of work, at least to start it up. Buying and holding domains, hoping for a few big sales, is an easier and more passive strategy but can take some luck.