The pros and cons maybe?
FTB is designed to provide you with a choice of your favourite mod pack downloaded, properly installed, kept up to date, and playable.
Getting a lot of mods to work together is not an easy chore, these guys should be applauded for this feat alone with the FTB Mod Pack.
MultiMC is a tool for installing mods one at a time into a playable game configuration call an instance.
Designed to work mostly with Forge API, the config files sometimes need to be adjusted to avoid block ID conflict, etc.
Quite often when you get an -instance- working well you can share it with your friends as a zipped folder or whatever.
So while FTB Launcher does all the complex work of mod building for you, MultiMC with a bit of study and research will allow you to build your own custom mod pack.