Depends on what your development experience is and what you may need from community plugins.  If you're not really doing development yourself and are relying on dropping in existing community plugins then you may want to use 1.8 as that's where most of the plugins are compatible at the moment.  If you're a developer and the project is going to be mostly custom stuff then definitely 1.9.

Any new from-scratch projects I'm working on are 1.9, and I've found that most of the 1.8 plugins work fine in 1.9 or need very little work to make them compatible.  For future considerations 1.9 would be ideal though as it will be supported longer than 1.8 and plugin development is naturally starting to swing in favor of 1.9.