One of the first things you need to decide is whether this is going to be the stakeholders in making this decision and who do you need input from vs. agreement from. It's often very useful to separate the two, so you can ask both groups relevant questions. It's difficult to find a solution that works for everyone if you're seeking consensus. There are couple of ways to address that - one is by being keenly aware of agreement vs. input issue, the other way is by narrowing the scope of the solution to either do less, or work for fewer people. Latter will lead to proliferation of systems, but that's not necessarily a bad thing - at the end of the day the worst possible scenario is one big system that doesn't work for anybody. By allowing multiple systems, you're getting the knowledge necessary to then come back and either integrate the systems or converge them.
Some of the other important points in making the selection:
Does it feel comfortable? One thing that gets a lot of people into trouble is that they go by the specs to make a decision. They tirelessly check off feature-by-feature against the competition. The truth is, if the feature is any good, it'll be copied by competition in no time. Go by the feel. Is this the kind of system you and your staff can see yourself using day in and day out?
Support? Can you relate to the person on the other end of the phone? If this is a system where you're going to have to spend some significant time with tech support to set things up and keep them running, call ahead, get a feel for what kind of people they hire to support you. Will you enjoy this interaction in the future?
Frequency of the upgrades. This can tell you a few things - how hard they are working on the product, how aggressive they are at being the best there is, how fast they will fix an issue you're having
Customizability. Everyone's business is a little bit different. Your edge over the competition is how well you engineer the processes, how much better your follow-up looks, how you utilize the knowledge capital you've accumulated. This implies that customizability is a key factor.. the system is a platform and more often than not you need to make it your own.
Reputation and viability. The good news is that SaaS space has been relatively stable when it comes to companies going out of business or other