Socialengine is more like our old Dolphin was - a CMS, while UNA is closer to a framework, allowing for greater flexibility in setting up a unique website. Another benefit is a more permissive MIT license. 

Socialengine founders started a couple of years after we launched Dolphin and a few years ago sold their business. I’m not familiar with the new owners, but they do keep the company going, which is a good sign. Anyhow, if you do end up using SE and later change your mind, we have a migration tool to transfer data between SE and UNA.