The LCP is a mean little gun with a fierce recoil that beats up your trigger finger. It's dead accurate, assuming you can train yourself to use the low profile sights, and at least mine had the pins start to walk out of the frame after a few hundred rounds.
The 642 is a lot better put together, equally accurate, and 38 spcl +P packs more of a punch than 380 ACP. Its frame soaks up recoil better, accuracy is a bit better.
They're sort of apples and oranges, that and the LCP is much thinner profile and more concealable.