We are CMIS, but connection probs mean that there is no dice - possibly our fault, possibly the county net connection 'solution', I am not the tech guy, and it is hard to know when 2 sources may be at fault - we have similar probs with the Fronter OES too.
We did have some 'interesting' times for the first 3 months, which resolved. We are not sure what currently causes our 500-student class and our roving extra y9 A2 course attendance, etc, however...
In my browsing of these blogs, people seem hacked off with sims, but with sims itself, and cmis seems unmentioned - I'm afraid on this basis I couldn't recommend one over the other - I think either have a good chance of becoming a replacement swear word in times of difficulty? Only a hunch, but I get the feeling that both Frog and Fronter did sims integration first, and that cmis integration is a little more up-and-coming. If all your classes have only one teacher, you may have a better time with cmis than we did, as that was a core problem for a while, however Fronter may have got this sorted natioanlly now? Good luck!