The King Of Efficiency: The Intel Pentium M 780
We would favor the Core Duo due to its two processor cores and the more advanced architecture. Yet the Pentium M is the more efficient solution for single-threaded applications and has a design power maximum of 27 W, while Core Duo can turn up to 31 W of energy into heat.
The Pentium M, based on the Dothan processor core, is a very mature product and in the long run will gradually be phased out in favor of Core Duo. Since there is only one Core Solo on Intel's price list, we expect that most Intel based notebooks will be based on dual core processors within a year.
However, the Pentium M can still be considered a good choice, since processor and motherboard prices will decrease, if they move substantially at all. An interesting thought would be to strip down the memory and hard drive of an existing PC or notebook and recycle it into a low-power PC. All you would need is a suitable motherboard with 855 or 915 chipset and Socket 479.