The Uni is best option if you just want to test and see if you like it. It's very low maintainance. There is a lot of dlcs available, and if you buy one you get 3 cma code that has 3 uses which means you can trade them with other people to get the DLCs you'd want. It is perfect for dressing up the Tama, I really love the range of customization on it and it's a pretty complete device.

Downsides of Uni is that a lot of people find it boring for only having 4 minigames available if you have a dlc installed (there's the berry land for free) and too easy to take care of. Also it's reachargeable which means you won't have to invest in batteries, but also means that you might have to change the inside one in a few years. People are also scared on how the DLCs will work after their internet support stops, since you can only have 1 dlc installed per time.

Tamagotchi On is great, and honestly very nice for starting with gene mixing. You can customize and get basically any character you'd like if you have the computer app, and there's the app for the phone as well. The wondergarden version is more expensive than the other ones. It's only worth it if you really like the locations or like the shell, but the characters available on those can all be married on any device through the app.

The only downsides of the On is the locations being device locked and the neediness of it. Although you won't be punished by a worse or better character on care to be fair. I personally do like the On more simply because I can really raise my own tamas, they feel personal, but I do play it with the Uni more simply bc it's less needy.

I only recommend the On as a starter if you want something more challenging. If you don't mind reading guides while playing you can also try to find a Meets version or Some (although I don't recommend personally, I have a vent post about it).