While I don't have a P's, I just want to third what Spacey and Popgotchi have said about the Mix: Definitely go for a 20th anniversary edition. I have the normal version (Spacy), and it started to feel limited to me after about two weeks or so of playing, since a decent chunk of the game is locked away unless you have multiple Mixes (two locations, 7 characters to marry, if I remember right, several foods, toys, and accessories, and a game). I have a 20th anniversary on the way, but in hindsight, I wish I had only bought the anniversary addition instead of the regular one first.
Gameplay-wise for the Mix, I fully agree with what Spacey said. The genetics and mixing feature is a lot of fun, and you can end up with some very interesting and unique combinations. Raising new generations and keeping the line going is entertaining in this version, since you'll always be surprised by what you end up with. However, there isn't much to do with them at all once they reach the adult stage; there aren't any stickers to collect or special features just for the adults. The only reason to keep an adult around is if you like how it turned out and want to hold on to it for a little longer. The number of accessories and wallpapers/"livings" is also very limited in the Mix. I've only seen 9 or 10 wallpapers, and I think I counted about 20 or so accessories. If you don't have access to the Mix stations in Japan, it's going to feel rather sparse. In other words, the customization in the Mix outside of the hybrid breeding system isn't all that great.
The result is that the Mix feels pretty passive. It's a great Tamagotchi to run in the background, since it's fairly easygoing, has a daycare system, and is relatively simple. If you're looking for a somewhat simple tama with a fun breeding system, it's great. If you want something with meatier gameplay and more to do with your grown-up tama, then I'd suggest taking a look at the P's.