We could have chosen to re-sell any of the market leading vulnerability scanning tools; we picked Tenable. Why?
In our formative days as a company we worked heavily with the penetration testing community. A group of highly skilled people, who only look at technical benefits (in our experience, pen-testers are not receptive to marketing messages). Almost without exception, they all use Tenable products!! Why? Because they do the best job of detection and have reasonably low resource requirements.
Building upon that technical recommendation, we like their products because they start small and inexpensive, scaling to the enterprise. They also offer both on-premise and hosted technology.
We find in Europe that people are less trusting of SaaS solutions, in particular for security information; so on-premise is important, and both the other major vendors have cloud elements to their solutions.