Cost should be much cheaper than smartmusic. The only thing I do not like about the musicfirst lms is the lack of content included. You can purchase a ton of items to use in music first, but that will make the costs significantly more than smartmusic.
If it had the library of smartmusic there would be no doubt as what to choose in terms of content and meeting the needs of your students.
I'm probably going to purchase music first for all of my students in band then buy just a few subscriptions of smartmusic for my really motivated band students that will be doing region/solo-ensemble/etc. Those that will push themselves beyond the normal requirements of class have more content in smartmusic.