I've written out a little list of the pros and cons of the colour Tamagotchis that I own, thinking about if I were new to colour Tamas. I started out with the iD LE, which I feel was a fantastic choice, although I had wanted to start out with a P's. That said, I wouldn't currently recommend the iD LE as a beginner colour Tama. Anyway, hope the following information is helpful!
iD
Pros: Nice and simple, easy to get downloads for (or can buy an iD Station for extra content)
Cons: Not a lot of in built content (one ten characters), only in Japanese
iD LE
Pros: In English, best in built games (in my opinion), some really nice features (e.g., growing seeds in the garden or visiting previous Tamagotchis for food)
Cons: Now expensive/hard to find, some in game glitches, need another device to unlock all 32 characters, can be difficult unlocking all the 'branches' of adult characters as Child growth is random (I never seem to get the child to unlock the 'bottom branch' of adults)
*This version is also available in Japanese, which is significantly cheaper
iD L 15th Anni.
Pros: Lots of retro characters in colour (feels more nostalgic), unique games, 32 in built characters, some really nice in built items
Cons: Only in Japanese, expensive to get new, Maze game is hard, need another device to unlock all 32 characters
P's
Pros: English patch, easy to get download (loads of official and fan made download content), lots of in built features, 'expansion packs' (pierces)
Cons: Need two extra devices to unlock all of the characters, pierces can be expensive/difficult to find, can be quite needy, can be repetative trying to unlock all the characters/places
*Generally quite a 'girly' Tamagotchi; this is either a pro or a con depending on your tastes.
4U+
Pros: Simple, graphics seem better quality than previous models, some of the best animations, three in built games
Cons: No English patch (yet, although it is being worked on; there is an English patch for the 4U), not a lot of in built content, difficult to get downloads (need an NFC compatible device and even then it can be tempramental)
M!X
Pros: A lot of replay value, four in built games, graphics are highly detailed (no icon bars means a 'bigger' screen), some really fun/cute animations
Cons: All in Japanese, no access to download content outside of Japan
*Game play for the M!X is quite different to other colours, you don't need to collect 'TamaTomo' (favourite items) or unlock places by raising characters. This is either a pro or a con depending on what you want from a Tamagotchi.
Hope this helps!