I think the big difference is in the support.
RTS gives you the software, but charges a yearly fee for each station that actually sells something (boxoffice, concession sales positions). You can install the same software at other places in your network for monitoring/reporting/scheduling purposes and not have to pay for those positions.
For the money, you get 24/7 support and all software upgrades, which can be relatively frequent. The company has made a LOT of improvements and adjustments to the system. They made many changes for me, based on the way a drive-in uses the software. The work they do to their system is available on their web site for all their customers. You can upgrade your software whenever you feel like it... no charge.
By contrast, Sensible charges full boat for the software, and you own it forever. However you'll eventually need to pay for support, if you think you'll need it. In the year or so that I had the system, they only made one upgrade. One might say their system was better from the start, but I think RTS responds faster to immediate needs and has a convenient upgrade path in-place.
When I was using Sensible, they hadn't added gift cards or online ticketing. Their system also had a limit on advance ticket purchases... something like a week. RTS will sell tickets for anything in its schedule. RTS also supports automated ticketing kiosks.
At the time, Sensible had only one person (the owner) for technical support, who wasn't always available. That may not be the case now... I don't know.
Both systems work... they sell tickets, produce reports and keep stats. The way RTS supports their system made the difference for me, since I have 8 sales positions, a kitchen pickup station, the database, giftcards and online credit card processing in my system. My whole business day is done in just a few hours, so this stuff has to work. A glitch can quickly put lines of cars out on a busy state highway, or lines of concession customers out both ends of the building... not good, but if it does happen and I can't figure it out myself, I need to know I can reach someone fast.
The fact that RTS owns theatres and actually uses the software they design and sell is a plus in my mind, too.