SpeedTree is a commercial software used in movies, games and other CG productions. It's the go to choice for plants creation alongside scans. There is a free educational version but it won't allow you to export the models. To do that you have to pay a subscription. You can, however, render the model inside the software to showcase it online and all the creation features are available to you.

SpeedTree has an entire company behind its development and, as such, it has way more features and it's more complex. The quality of the models is also way higher. You can export photorealistic trees with millions of polys or low poly trees for games.

If you only need basic trees, TreeIt is enough but if you want to create any kind of plant without restriction, even fantasy or alien species, SpeedTree is the way to go.