Arduino guy here.
For your everyday reefing hobbyist the Apex wins hands down. Now I cannot speak of the Reef Angel and its features but the Apex is amazing in what it can do. Having an IT background I can see the benefits of open source anything, HOWEVER, open source brings a niche market, almost fanboy-ish. Once you step into a proprietary market, you enable the masses a uniform platform. Apex has done it right and (not knowing the specifics) has won the market share. I would love to see an open source controller take over. That would mean manufactures turn the curve and start producing their products to be compatible with 2 platforms (ie. iOS vs Droid platform cell phones). This would also drive prices DOWN in the controller market, making it more affordable for all of us to own one, it would also drive prices down in controllable accessories.
Apex Fusion is the next step in remote capabilities. Multiple users inside the dashboard is something I've been wanting to see from them for a while. From the video they are also enabling a nice remote monitoring feature. It sounds like Neptune will email you when you have a power outage and/or internet blip. This is something that has plagued us all. Your Apex will email you if something is off, but not when the unit itself is off. I've been working on trying to provide a solution that would be able to do that. Not saying that Apex Fusion is the golden apple in a bunch of granny smiths, but this is something your everyday hobbyist can rely on to protect their investment. If Apex does this right, they will DOMINATE the controller world. I can already think of 100 questions to ask them about this solution because us IT dudes battle this day in and day out. I would imagine a subscription based service for this, due to the bandwidth requirements alone. I would love to see the new firmware code and how its written. I've uncovered very very few bugs with the Apex system in my experience, you move to open source and every person will experience a different issues due to coding, hardware revisions, etc.
Sorry for the long post and ramblings
Sources: Own an Arduino, setup a bajillion Apex controllers.