I was involved in migrating from SIMS to Facility CMIS about 5 years ago. That was pre-SIMS.net and, cost aside, it was a no-brainer - CMIS was clearly the superior product.
With the advent of SIMS.net, the two are much closer together. This means you really have to think carefully about what you want to achieve by switching systems and whether it is worth the upheaval.
As has been mentioned above, Facility is strong on configurability, so you can use it to support you school systems and processes rather than having to change your systems and processes to match the software. However, this means the learning curve is far steeper and it's not as user-friendly 'out of the box' as SIMS.net.
To make a success of CMIS, you need to make sure that the staff and management have the appropriate time, skills and motivation. That really means a dedicated Data Manager working purely on MIS, plus a management team who know why they want to move to CMIS and who will have a hands on involvement in managing the implementation. CMIS will be more work, not less, but is much more powerful if you get it right.
Where schools have regretted moving from one system to another it is usually because the management team thought - 'we don't like SIMS / CMIS and it's causing us trouble, let's try something else'. The decision should be made in terms of specific things the school wants to achieve, and the knowledge that your current system definitely can't help you achieve this and the new system definitely will. It's all basic stuff, but you'd be surprised how many schools don't think this through properly.