I prefer each of them equally for different reasons. The plus colour runs like a connection but in colour, with the addition of weather, seasonal animations, character specific animations (characters have their own items as well as having universal items). I love how this tamagotchi really isn't meant to be played for one or two generations it takes a lot of time and actual effort to unlock everything. This is also the only tamagotchi in colour that you can have an oldie on. A lot went into the world building in this tamagotchi and it really shows. It's the little touches. There are no downloads but there are passwords. Tamagotchi's die, you see a shrine and the death god with they do die.
The 15th anniversary IDL is an IDL and naturally, I love it. It runs pretty much the same as your every day IDL. There are 32 characters that you can obtain and you need all 4 tamatomo stamps to unlock destinations. 4 adults can only be obtained by connecting (to ID IDL or Ps all work). We can still adopt pets and plant trees. There are of course different characters than the IDL, and different wallpapers and games and items. There is the option to add to this tamagotchi by downloads (games, wallpapers items foods clothes). We can dress up our adults with clothes and accessories. Tamagotchi's don't really die they run away and leave you a letter
To compare them is to compare apple pie and apples. Both are good both have a main part of being apple and both are good options. Both taste good both can have different things added to it to make it good, and both are perfect just as is. So it really all falsl on you of what you like and why. Do you want a simpler one or more complex, do you like running for a short period of time or a long period of time. But again they are both great.