Digimon didn't try to say that there were a specific set of species in the world, then expand it without any real explanation in the way that pokemon did. Also, Digimon was about working together to save worlds, not making animals fight for fun. I feel Digimon had the better premise and didn't do things that made its later series dumb.
Hence why I really like Digimon. They're digital beings, and not necessarily flesh and blood creatures. They're fully sapient and they choose YOU to help them save multiple worlds, rather than capturing animals against their wills and making them fight each other for sport.
And here I am with my Charizard avatar.
He broke free of the shackles of his Pokemon "Trainer". Perhaps his brothers Squirtle and Ivysaur might do the same.
I honestly think Digimon's failing is that it seems too complicated, compared to the more structured, orderly simplicity of Pokemon. At least -I- felt the series (the games, and the show) was very complicated compared to Pokemon as a kid and hard to follow. I think if Digimon was a bit more structured and clearly explained it would have had a lot better of a following.
Also, one of my pet peeves of Digimon is the inconsistency of its Digivolution lines. I got really confused as a kid when looking through the completed Digivolution map in Digimon World 1. And I was super pissed when my Agumon always turned into a Centurimon instead of Greymon. Sometimes I got lucky and got Tyrannomon.
It got to the point where I used a Gameshark to give myself Grey Claws. I wanted GREYMON DAMNIT.