New... There is no question.
As bwana mentioned, the warranty will still be valid (which is a HUGE advantage). Aside from that, you will know how a new notebook has been treated (you are the only one that has owned it), and as such, you can be assured that there has been nothing dodgy done to it (modifications, etc that may blow up in your face later).
What are the specs of the second hand notebook vs the new one? No offense intended, but if you don't understand the way performance is measured, you may fall prey to good marketing:
http://en.wikipedia.org/wiki/Megahertz_Myth
Many people fall for this one. I still have people telling me that their Pentium 4 @ 2.8GHz will kick a Core 2 Duo @ 2.0GHz. They think clock frequency means everything. Then you have people claiming their Radeon X800 512MB will beat a newer card with only 256MB memory. And don't forget the people who don't understand why one would pay more for less memory (for example with the 8600GT that comes in a 256MB version and a 512MB version, the 256MB version having DDR3 vs the DDR2 in the 512MB version). Similarly, you have people who think a GeForce 9600GT will beat a GeForce 8800GT because the model numbers suggest that.
This stems from, at least, two main things:
1) People don't understand that performance should be measured as a whole. For your computer to perform well, it is necessary for all components to work well together.
2) Marketing lists specs that are usually not the only relevant information. As most people couldn't be bothered to read up on the products, they take the stated specs as being the best measure of performance.
What I'm getting at here, is that perhaps the second hand notebook has specs that SOUND better, but are actually not better at all. Remember, chipset plays a large role in notebook performance. And a Core 2 Duo based on Penryn may run at a lower clock frequency, but will still outdo a Core 2 Duo based on Merom.