Total Plants are SpeedTree items and they cannot be exported. This is a restriction imposed by the vendor of SpeedTree.
Unity has their own version of SpeedTrees included:
https://unity.com/products/speedtree.
As to your question of what is a better way to create your environments, that depends on what you want to do.
I do everything in iClone as I have used it for a long time and have a lot of assets. However, these have become quite expensive, so Unity may be a better deal.
Since you are new to both, you could also look into Unreal as there seems to be more support for a pipeline with Unreal and there are very knowledgeable users on the forum.