I am holding out until I hear demos and walkthroughs about MSB, and AudioBro has given great walkthroughs so far with their previous libraries. I really want to have the ultimate brass library in my hands, which does ppp -> fff, can do divisi, sounds great out of the box, has good selection of mics, can do both slow and fast well, and has smooth legato.
What's intriguing about MSB is that it can do divisi (basically I can load one patch, play solo lines, or a2, a3 or a4 passages), it has 30 recorded instruments, you can change attack type for any articulation (similar to Chris Hein's libraries), lots of short articulations. Also it has sizzle knob, which I think gives brassy, forced tone, or dark, mellow tone, which is wonderful to have, because I've understood that playing cuivre (brassy, forced tone) doesn't equal to playing loud. That's what most libraries do: play loud and it will sound brassy.
Being able to change the color of the tone gives more control to the user. Can't wait to hear, how the library sounds.
CSB on the other hand so far looks like it could be my favorite brass library, when considering just the sound. The library isn't giving that much control to the user, but it doesn't need to, because the library seems to have very clever scripting under the hood. I'm not sure though, how I would use it e.g. for horn chords: Would I use the solo horn on 4 separate midi tracks, or the 4 horns patch in a chord, and how would it sound?