It really falls on what you want to get out of it. Are you looking for a complete playing experience where you only need the one to enjoy all of the content? Then go with the mix and enjoy mixing characters and see some really old faces all in colour. I really enjoy the mix it's a new look at tamagotchi and the anniversary is complete (in that you only need the one unit to enjoy all of the features)
The IDL is one of my favourites and I still love it more than I love the Ps. It was my first colour. I love the growth chart in how there are three branches (three teens each teen and evolve into X number of adults). Plus the gardening feature, and adopting pets (I think there are 10 different ones). In my mind, the IDL is really underrated. Fair warning there are 4 special adults that you can only obtain by connecting to other tamagotchis (ID IDL and Ps all work). If downloads are also something that you have access or interest in I think the IDL handles those well, they can add to your playing experience but don't define it. In that, you can get full enjoyment without them but with them they just add to it.
In the end it all falls on what you have interests in and your playing style. If you like the Ps you'll probably dig the IDL. If you of the mind that the Ps is the end all and be all and nothing will ever compare and without the "goal" of getting all tamatomos to keep you interested than the mix may not be for you. Again is falls on you and your interests and what you like.
