I use both system extensively amongst others, I have been using Watchout since version 2 and Pandora's box since version 4. I build workstations and solutions using both system. I mapped planes, buildings, hard sets and made ultra large projection for the past 20years using both system, well, Pandora's Box got in the mix the last 5 years actually. I program Coolux Widget Designer also and will get into Watchnet as soon as my schedule gets less crazy.
Watchout is a real-time interactive compositing software, think after effect in realtime. It was conceived and evolved with a video workflow in mind. It does 444 and uses various compositing technique which makes it far more advanced than Pandora's box as far as image quality goes, on the other hand it used to be slower and far less responsive than PB, that changed with version6, PB still has the edge in interactivity especially when tracking is involved but it is not as clear cut as it used to be. Watchout use the progressively complex approach, means that when opening the app everything is super simple and it will get as complex as you want it when you want it, but it starts simple. Watchout is almost all contained in the Production app, Watchnet, the dynamic image server and iOS apps are part of the ecosystem but aren't needed for normal operations.
Pandora's box is a media server, think videos used as lighting devices. It was built with a lighting workflow in mind. It is very responsive, extensively programmable and quite stable and versatile, in my opinion way more versatile than Watchout. It used to be the only package out of both to do 3D mapping. However it is at most 422 therefore keying and blend effects are rough, display aliaising and overall lack the finish watchout has. Also it is a layer based license, so you will have to choose how many video layers you will need when buying the product, a faster machine does NOT translate into more layers. It is either non-linear or very linear, basically you either program with a lighting console or with the included timeline tools which are very limited because of the license method. So for example if you have a sequence using layer 1 and 2, your other sequence cannot use these layers or the content won't be shown if present at the same time, greatly limiting non linear shows using the provided tools. Compositing works in a very weird way, you have access to blend effects for example but it can resize other layers, or obfuscate some, it will treat all layers under the effect and not only those you wish. No folder tracks, no groups, just basically the most advanced playback machine there is, not compositing, even though it does a few compositing tricks. Pandora's Box uses the versatile approach which means that every options are there and are in your face, nothing is really hidden, it makes the software quite hard to learn at first but it gets quite powerful as soon as you get it, you have to make it simpler on purpose but the Widget Designer makes it very easy to do so once you learned it. Pandora's Box is an ecosytem, you have the manager to manage servers, players, medias and timelines, the warper to warp content, Widget Designer to program interactivity or user interfaces, an app to monitor and start servers and players, and a floating menu to access various options like resolutions and others.
That said the biggest difference in the end as a user between both platform is service. Watchout has outstanding service, forums are monitored and people will easily get help here. When communicating with the people at Dataton they receive criticism and suggestion and consider them genuinely. It used to be different back in version 2 with their awful email forums but it changed a lot and now are an example of how to do stuff and consider your customers. Coolux on the other hand are very proud and easily offended. Their distributor here suggested I write to them and exposes the problems I had with the solution initially so I could get help on how to do stuff or at least know what's up. I received an awful letter of complaint where my distributor was being asked to talk to me because how dare I question their product and say it's not optimal. seriously, I am not joking, I was treated like a child by overproud support and have been reprimanded. After that I still kept using their product and even went as far as going to Germany to receive private teaching with my colleague for 4 days. I had a great time and they were very nice and helpful but my initial contact with them was just plain awful. After that visit we had to call support because of some serious issues where their warper kept loosing and corrupting our warp files but they don't work past 17:00 in Germany so screw us. When we finally got to them they told us completely unhelpful stuff and were basically of no help. Now that they have been bought by Christie things are just... not so alive, they released version 5.9 and version6 is told to be a fantastic upgrade went it will come but other than a patch and rumors things don't move much.
If you need more info, PM me I will be glad to provide assistance.