For me, outside of the obvious answers, the most important factor is scalability. I don't want to end up in a situation where a SaaS isn't able to tolerate my required workload anymore. This causes a lot of unnecessary stress and time to quickly switching to a different solution.
I am actually launching my own SaaS right now on PH: https://producthunt.com/posts/ezml. Iwould really appreciate the PH community's feedback on it. Especially with relation to the factors stated in this discussion :)
First, think about how it satisfies your needs and the problem it resolves. Then consider the cost, integration potential, customer service, and frequency of software updates.
ezML