I own a P's and a 4U+ and I actually really enjoy both.
The P's has more built in content, even with the downloads/pierces/etc.; the 4U/+ was designed as a 'bare bones' Tamagotchi, with the idea being that you could customise it with downloads to be whatever you wanted. A lot of people really didn't like the 4U/+ for this reason, but I think the graphics are lovely and it has some nice features and animations. I think it helped that the phone I already had has compatible NFC, which meant I have been able to fill it full of downloads.
The P's is still fun without the pierces, although if you do have access to downloads it feels limitless as to what you can have on there (Mr.Blinky has downloadable characters and English languages versions of a couple of the physical pierces).
If you have access to an irDa or NFC device then there is also an English patch for both the P's and the 4U (but not the 4U+). I think some sellers do sell the P's/4U ready patched though.
If I remember correctly, the difference in content between the 4U and the 4U+ is that the latter has two new characters, different games and extra 'personality stages' (when you have an adult Tamagotchi if you do certain things, e.g., bath X number of times, they will transform into one of their 'personality stage' characters).