It mostly depends on your needs. The core i3-380UM is actually quite a slow CPU at 1.3Ghz. The UM series cpus stands for Ultra Mobile, these are slower than even the ULV (ultra low voltage)versions of the chips. If you are looking for awesome battery life for a laptop that will be used for office applications and web browsing this would be fine.
The core2Duo is more of a workhorse, there is little distinction between the desktop and laptop versions of these chips, so if you want more CPU power the 2.5Ghz core2duo would be about 50% in any test you can put it up to.
i3-380UM 1.3ghz great batter, low performance 2 cores 4 threads, no turbo boost
C2D-P8700 2.5ghz higher energy use, older tech, faster 2 cores 2 threads no turbo boost