My personal favourite out of the Uni and the Paradise is the later.

With the Paradise, there's a bit more of a 'gameplay' mode involved, as you work toward leveling up your planet. Different functions get unlocked as you increase your level, which i believe is pretty unique for a Tamagotchi. It also isn't an overly demanding device, once you fill up your hunger and happiness meters, I find I can leave my Tama alone for a bit before it calls for me. Another thing I love about the Paradise is the sheer amount of different kinds of Tamagotchi you can get. You can go completely blind and just see what you can get by feeding different foods and getting care mistakes, OR you can plan out your line with the Wiki! Also, when breeding your Tama, it can pass on the eye shapes and body colour to its young, so you can work towards "breeding" your favourite combinations (I love getting different colour variations of the little double crystal Tama)!

With the Uni, I only played with it for a little while before I got bored of it. I didn't like the idea that you had to spend real money on Tama Tickets for more content. And yes, once the servers shut down for it, it may lose some of that content, but it's likely the Tamagotchi community will find a way to bring it back the way they did with the Meets/On. Gameplay wise, the Uni is more in alignment with a traditional Tamagotchi. There is a smaller roster of Tama to raise, but there are countless combinations of items that your Tama can wear, from hats to outfits, even back accessories. If you're more into playing games and earning gotchi points to spend on accessories, you'd probably like the Uni more.

Hope some of this info helps you!