Depends on what is important to you. If you want portability and don't need to write DVDs with an internal drive get the 12". Get the 14" if you need a bigger screen (it's the same resolution as the 12" model, mind you, just bigger pixels), a bigger hard drive standard (you can upgrade the 12" model's 40GB HD to 60GB at the Apple Store for $50 extra, though), and/or internal DVD writing capabilities (an external burner on the 12" is doable, too) and don't mind the extra weight or $300 cost.
Gee...sounds like I'm baised towards the 12" model...
Voch