Im digging through the forum for another game and found this thread, and im only replying cos i think theres more worth adding.
IMO theres more than just 2 parts, and neither a yes or no for 'is it an MMO'.
Squadron 42 is a completely story driven campaign (where friends can jump in and help you, if you wanted). It ties into the persistent universe granting you citizenship with the UEE, you can skip it and just be a citizen (fewer perks, but afaik no 'off limits' stuff) or you can skip it and be a pirate type (buy forged citizenship to sneak into UEE areas etc). But it should be a 50hr (25hr+ gameplay, the rest being interaction/story etc) game capable of standing out amongst any other SP game.
Star Citizen is an MMO which isnt exclusively an MMO. Its the online persistent universe, but you can opt to play it PVP (good opportunity that the people you see in space are players), PVE (the people you see will be AI/NPCs not players), or somewhere in between. Im not sure you can do 100% PVE/NPC so you'd never encounter another player, but i dont see why it wouldnt be possible.
Also, its worth pointing out that players will make up just 10% of the characters in the PU. So if you go full PVP, its not like every single person will be a player, griefing and ruining your experience. The 10% may include people on planets more than in space, dunno.
What it does mean is if you want to play in this evolving world, without being bothered by others, you probably can.
On top of that, there will be the ability to host your own server, with your own rules, mods etc. Someone creates a Firefly mod, you're free to play it with friends if you want. Its a much smaller universe, and wont be authored like the PU, but it you host it and dont let others join, i dont see how people will be in there etc.
For stuff like this, or other questions, you really wanna ask in the SC thread cos you'll get more people who can give you a fuller answer. Its pretty much a 'play it how you want to play it' game.