Guess that is good for us end users to have two platforms to choose from!
So for you that have not read about either platform-
The basic idea is to be able to upload tracks and people can join in and add to tracks or vice versa. There is some differences in-between Bandlab and Songtree which is a question of preference. I think that the idea is very cool though and it can lead to long time collaborations and must be very exciting for musicians that need more instrumentation and not having access to other musicians...
To read more about Bandlab's Treeview: