First, let me say that Sourcetree is an excellent application, too. However, most people that chose Tower over Sourcetree prefer our approach in regard to user interface, workflows, usability, etc. A visual conflict wizard, a new service account manager, automatic stashing... Tower goes a long way to make using Git both as easy and as comfortable as possible.

You can see my comment elsewhere, but I've been messing around with Tower 2 some more, and I'm actually finding it pretty confusing. It seems submodule support is still flawed. For example:

https://www.evernote.com/shard/s218/sh/755a2a30-db64-4378-bc...

I'm not sure why it's showing the submodule folders as something I need to commit.

I prefer the combined view that SourceTree has of changed files with your branch history. In Tower 2, they seem to be separated.

I do like the breadcrumb trail for going into submodules though. SourceTree opens it as a separate window, which I find annoying.

Mostly a matter of preference. Several tower users on our team and a SourceTree user. Try both, take cost into consideration, and go with what works for you.