I'll go against the grain here:
If you're doing more than very basic stuff, the core i5 will be faster, and get better battery life (assuming more than an "idle" workload) as it will "race to sleep" faster (it is more efficient in terms of instructions performed per watt).
Also, if you want to run encryption on disk, the Core Is have AES encryption acceleration in hardware (they're almost 30x faster than a Core 2 at this). They're also WAY faster at transcoding/encoding video.
The i5 will be more future proof. You'll also get thunderbolt, which means you can use a thunderbolt display as a dock, down the track (and get gigabit ethernet, multiple USB ports, etc).
I'd take the i5 no question - your expansion options on the c2d are limited to USB2 - no gigabit ethernet, no high speed disk, no external video, etc. Might not need it now, but if the price difference is bearable, get the i5.