They are essentially the same pistol. The P1 is a P38 with "P1" stamped on it, and built for German government use. Walther did some upgrades to the P38 at some point, so not all of the P38's out there will have them (depends on when it was made). AFAIK, all of the P1's will have the upgrades.
Some P38's will have some collectible value, while virtually none of the P1's will. I would bet than none of the one's you are looking at are of the collectible variety.
They are great shooters, and the P1 I have is one of my favorite pistols. If you can get a really nice example for under $250, jump on it....