The Sentry stemmed from over a decade of doing traffic analysis (from a security perspective) for local small and mid-sized businesses. We had a great conglomeration of tools (ntop, cacti, snort, some custom connection indexers, etc), but it was almost impossible to use effectively if you weren't the guy who made it.
We wanted our clients to be able to monitor their own network, so we set about making the Sentry. The primary feature of the Sentry is usability. Traffic analysis is typically hard. There's no reason it needs to be.
We would love to have any feedback you guys are willing to offer. Hackernews is famed for it's frank discussion. Bring it on.
1. Use case: Moloch, with it's focus on PCAP, is a cool network forensics tool, to be sure. Our focus is not only on security, but also on performance and troubleshooting. We use the traffic data as a foundation for integration with IDS alerts and latency (and jitter) data. The ability to pivot between views and visually correlate this data is incredibly (and surprisingly) useful for IT Staff who want to know at a glance if things are correlating or not. So I would suggest that Moloch is a little more limited in it's scope, at least in it's default configuration.
2. Usability: This ties in with the point below too. Our focus is making network analysis approachable for the average IT guy. Keeping things light, visual, and easy to use is super, super important to us. It's really the top item on our list under, you know, having correct data.
3. Maintainability (or SLAs, or whatever you want to call it). When I look at Moloch, I'm reminded of Zimbra, the free email server. Last time I installed Zimbra, it involved installing a lot of different stuff, configuring it all, and groking a LOT of documentation for different components before it was working to satisfaction. This is perfect for a certain market segment, the IT guy who has the time and energy to - not really roll his own - but to be really involved in everything that's going on.
the Sentry is not for that IT guy. We market to IT staff who don't have the time to install and maintain a bunch of components for a single tool. They have a security / network vision need (sorry about the buzzword) that needs to be filled, and they don't have a month to stumble through a complex linux install. Don't get me wrong, I personally am all about complex linux installs, I love getting into things myself, but this product is pre-packaged, with batteries included.
Hope that answers some of your questions!