I have both a 4U+ and all three M!X versions, and I enjoy them all! I think it depends on what you are hoping to get out of another Tamagotchi. The 4U/+ has similar gameplay to the P's (raise a Tama, collect TamaTomo, marry repeat) whereas the M!X is completely different.
The 4U/+, if I remember correctly, was marketed towards young adults as opposed to young children and so I do think it has a more 'sophisticated' feel to it. It's also one of the least needy Tamagotchi models (I've forgotten to pause it before work and come home to find it still alive, albeit unwell) to help it fit around busier schedules. It seems to be that the biggest problem people have with the 4U/+ is the lack of inbuilt content; for example, there's only ten/twelve inbuilt characters to raise. The idea of this was that it was a 'bare bones' Tamagotchi that you're meant to fully customise with downloads, but not everyone has easy access to these as not only do you need an NFC device you need the right kind of NFC. My phone is compatible, so since day one I've had access to the downloads for my 4U+, which may be why my personal view is a bit more positive than others. I really like the 4U+; I think it's got some really fun animations.
The goal of the M!X is just to raise and marry Tamas. Some people love this, some people find this tiresome. I like to pick it up for a bit, raise a few generations then put it down again and focus on a more goal orientated Tama (e.g., the P's). I got my Anni. M!X for Christmas so I am actually running most of them at the moment so they can connect, unlock places, etc. The M!X easily has the best graphics (the icon bar is 'off screen', meaning the animation window is larger, meaning more detail) and has some utterly adorable animations. It does has some flaws though, such as I find earning GP a bit of a grind at times, although it does have four in built games (only two at the Arcade though, which is annoying when you're trying to earn GP as if you're playing a game at a location it takes you back to your Living Room, rather than the menu, after each play through). It's not without it's flaws though; the Spacey/Melody version needs to connect to two other M!Xes to unlock all of the locations; if you don't, you could potentially not have access to six of the twenty-seven characters to marry; this is what made me put mine down originally as I found it highly frustrating (all unlocked now though). The Anni. version doesn't have this priblem though; everything can be unlocked on the one unit. The offspring also don't alwyas mix fantastically; on my Melody ver. I had a purple Kikitchi and his daughter was a dark blue Kotohimetchi, despite both being several generations in and should have had traits from a number of different characters.
I like both, to be honest, and I think it's hard to say whether the 4U+ or M!X is 'better' as the gameplay is so different.
