I have an iD L (E and 15th Anni.), P's, 4U+ and M!X. I have an iD in the post, but as I haven't used it yet I can't really comment.
The iD L is my favourite model; it has loads of inbuilt content and I prefer some of it's features and characters to the P's and the standard version has my all time favourite Tamagotchi games. However, it's only available in Japanese (unless you get the highly sought after and expensive English version). There are three different versions you can get, each with different games and characters; the standard iD L, Princess Spacey Ver and the 15th Anniversary Ver. I have the latter; I personally think it's adorable and, as it has loads of the old characters, has a nostalgic feeling that I haven't found in other colour Tamas. That said, the 15th Anni. is quite expensive to get new (second hand ones can be reasonably priced). I've not got the Princess Spacey ver., but it's the same model just with a couple of different characters and new games; it does go quite cheaply (in comparison to others) though. The iD L was my first colour Tamagotchi, so I would definitely recommend it as a 'starter' colour.
I do also really like the P's and, although I prefer the iD L, would recommend this over the iD L as a first colour Tama. This is mostly because you can get it patched into English; even if you don't have a compatible phone/dongle, some people will sell P's (even new ones) pre-patched and with downloads at your request. The addition of the pierces also increase the replay value of the P's; between the inbuilt content, the pierces and the VDPs (downloads), it feels like the gameplay is endless! The P's is very similar to the iD L in terms of gameplay; they just have a few different features (e.g., growing seed in the garden on the iD L or the garden visitors in the P's) and while the pierces aren't necessary, they are fun. As I said, I do think that the P's are a great way to start.
The 4U/+ is very limited in built in content, but if you have a compatible phone (which you say you do) I think they're great fun. As you said, they're also some of the cheaper colour Tamas. The 4U+ has a little more in built content (two more characters and extra features such as growing food at the farm), but the 4U can be patched into English, so bare that in mind when choosing which to get. Hopefully there should be a 4U+ translation at some point, but not at the moment. I enjoy the feel of the 4U+; the graphics seem a little sleeker and the whole thing feels more 'sophisticated' than the previous models (it's target audience was young adults as opposed to children). A lot of people don't like the 4U/+ due to the limited content but the idea was that you could customise it to be your 'perfect' Tamagotchi with the downloads. As I have a compatible phone this works for me and I actually think it's great. It's also less needy than the iD L and P's.
The M!X isn't my favourite Tamagotchi, but that's partly because I've been unable to unlock everything yet (waiting to get another one!). It's a bit difficult to say whether or not this is a 'good' Tamagotchi to get as it's gameplay is so different from others. As I'm sure you're aware, the premise of the M!X, rather than to unlock all of the built in characters, is to breed your Tamagotchis and mix their genetics to create new Tamagotchis. It is surprisingly fun, but I personally feel limited by who I can breed with as I'm missing five of the characters. I also don't enjoy the games on the M!X as much as I do on previous models; that said, there are four built in games as opposed to the normal two (although it is possible to have up to five games on a P's; two built in, two download and one from a pierce). Again, as I've not unlocked everywhere I haven't played all of the games yet. However, the menu bars disappear meaning the 'active' screen is larger, which allows for much nicer, more detailed graphics. The M!X is a delight to look at. The M!X is also in Japanese and it looks like it'll be staying that way as (currently) there's no way of getting downloads onto the M!X outside of Japan.
Hope this helps a little bit! Feel free to ask more questions if you like!