Hehe. I highly recommend visiting this site HERE for information regarding the vintage Tamagotchis. This page HERE under FAQs talks specifically about the differences between the P1 and P2. There are some character and cosmetic differences but the game play between the two is pretty much the same.
Some designs are worth more than others and if they come new-in-box, you can typically see the bids go up to $20-25. I'd also pay the same amounts as phnx0313 mentioned.