I have all three M!Xes, two iD Ls (E and 15th Anni.) and a Plus Colour. I enjoy all of them in their own way.
The M!Xes are the most 'unique' Tamagotchi I've had. As I'm sure you know, with the M!X you marry characters off to play with genetics. The graphics on this model are probably the nicest of any colour Tamagotchi and, without the icon bar permanently on the screen, there's more room for the sprites. it looks pretty, every generation is something different and, as I said, it's the most unique in terms of gameplay. However; I got frustrated with my Melody and Spacey versions at first because I could not unlock the destinations (need to connect to three other M!Xes to unlock Mermaid Palace, which you need to unlock Gozaru Village), which meant I was missing about 25% of the characters. It meant I put them down quicker than I wanted to. Since then I have been able to unlock everywhere and I am loving it so much more. I quite enjoying running one M!X alongside one of my older Tamagotchis now. The Anniversary version is better in this sense as you can unlock everything without having to connect to other Tamagotchis (although I personally feel that the Melody/Spacey versions have better games). There's no 'goal' on the M!X like there is with the other models (as in, no TamaTomo stickers to collect). It's hard to compare the M!X to previous models because if you like the idea of virtual genetics then you will probably love it, but if you want something more goal orientated and less 'sandboxy' then it's probably not for you.
The iD L (E) was the first colour Tamagotchi I owned. It's a good place to start and is one of my favourite models. There's a lot of in built content (I think there's 32 in built characters?) and has some lovely features such as gardening and visiting ancestors that are absent on most other models. I also think the iD L hands down has the best inbuilt games. I played with this Tamagotchi for over a year before I started buying other models for it to connect to. I also enjoy the fact that the iD L (and P's and 4U/+) feels more like a 'game' than a 'pet' in the sense that you need to collect all the TamaTomo, which in turn unlocks new content for the Tamagotchi. When I get a new adult I love trying to figure out their favourite items based on their personality (e.g., if they have flowers as part of their design I try all the gardening themed items first) and sometimes it can take me days. It's really cute, although I personally feel that the 4U/+ and M!X both have much more adorable animations. The growth system can be a little frustrating (there's still characters I haven't had yet as I can't get the right child character) but overall it's good. You can also get downloadable content for the iD L, which you can't for either the M!X (at least outside of Japan) or the TMGC+C. The 15th Anni. iD L runs exactly the same as the iD L (E) but has a few different characters (characters from previous models for a more nostalgic feel) and wallpapers/items/etc. with more of a 'celebration' theme. The games are also different (very fun but very hard). If you want to know any more about the differences I'm happy to explain.
I only got my TMGC+C yesterday so I can't fully comment on it yet. I do know that it's got a lot less content than both of the above, but it's quite sweet in it's animations and what it lacks in graphic sophistication it makes up with charm. I'd say this one also feels best in the hand for me and the shells, although simple, are really very pretty in real life. The games are fun and there's a gardening feature (which seems to be more in depth than the iD L version), which I'm looking forward to using. I've not yet unlocked everywhere yet, but so far I'm enjoying it. there's also no TamaTomo to collect on this version, but there are 'Happy/Lucky Items'; I'll be honest though, I'm not entirely sure what they are right now! still figure this Tama out!
Hope this helps a little bit and that you get yourself a new Tama soon! =)