I have both a P's and a 4U+. The nice thing about both of these models is that both now have English patches (well, the 4U does, the 4U+ does not). I think it depends what you want from the Tamagotchi.
I felt the P's felt more like an updated version of a classic Tamagotchi, whereas the 4U/+ felt like something different, a bit new. This might be because, to me, the P's is very similar to the iD L; the 4U/+ changed the position of the menu icons and was a bit confusing for me to get my head around to begin with. Both the P's and the 4U/+ have loads of extra content, but how you get it differs; the P's have the pierces, which are sort of like 'expansion packs' that you plug in, while the 4U/+ has a load of content that can be downloaded through the app with a compatible NFC phone/tablet. A lot of people find the 4U/+ too reliant on downloads, with only the 'bare minimum' preloaded onto the device; I personally quite like it, as it sort of allows you to completely customise the Tamagotchi (but then, I already had an NFC compatible phone before I bought my 4U+). The P's has more built in content and it's easier to get the extra content, as you just buy and plug in a pierce; however, the pierces do seem to be constantly going up in price. The 4U/+ is also cheaper than the P's despite being a newer model.
I also think the 4U/+ has crisper/cleaner graphics and personally prefer the shell design (I have the 19th Anniversary version, which is possibly my all time favourite shell design). You can also buy new faceplates for the 4U/+, which doesn't add anything to the gameplay, but can look really nifty (I'm a little obsessed with faceplates); that said, you can get custom pierces for the P's, too, that have no content but look pretty. For me, I'd also say that the 4U/+ has 'girlier' content than the P's; however, this may just be how I set mine up (I set my 4U+ up to be very girly looking) or just that there's more choice of wallpapers/icon menus/etc. on the P's, so it doesn't have to be girly.
I actually quite like the 4U+, although I've neglected mine quite a bit, as I got it only a few weeks before the M!X came out. However, when I'm bored of my M!Xes, I'll probably go through and play with the 4U+. Another thing I like about the 4U/+ is that you don't need multiple devices to unlock all of the characters like you do on the P's (to unlock all 32 characters you need at least three P's).
In short; the P's has more in built features and is not reliant on downloads or extra content. The P's also has pierces, which can be super fun. The 4U/+ has less content but if you have access to the downloads, allows you to 'full customise' your device with the items, clothing and Tamagotchis you want. The 4U/+ also looks a little sleeker on screen. Both have girly content, the 4U/+ is possibly slightly girlier, but the P's has more choice.
Hope this helps a little! Feel free to ask me any questions you have about either or!
