Difference between Module and Add-on

I am hesitating between a Module (Lost Mine of Phandelver) and an add-on (The Sunless Citadel).  Is there a way to know the exact content of each? I know I have to create a new game with the Module while I can just "add-on" the add-on, but I would love to be able to compare the content. 

The Sunless Citadel has 3 Dungeon Maps, about 30 Stat'd Monsters with tokens, and around 50-60 handouts for plot, puzzles, items, etc. The Lost Mine of Phandelver has 7 Dungeon Maps, about 25 Stat'd Monsters with tokens, about 25 Stat'd NPCs and Uniques (some with tokens), 60-70 handouts for plot, puzzles, items, etc. I've never played Sunless Citadel, but my feeling is you would consume the full material in about half the time it takes to do all of Lost Mine of Phandelver.  YMMV.  =D

Thank you Aaron, your answers are appreciated.  I'll probably go with the add-on, since I want to create some content myself and customize the game along the way. 

Once you have them in the game, neither restricts you from modification.  I'm running a version of Lost Mine of Phandelver (I call it "Lost in the Minds of the Phandelvers") which is wholly unrecognizable compared to the base module. =D

I will agree with Aaron on the "either". Because I happen to enjoy the module, I'm currently running 3 Curse of Strahd settings. I don't call them games, since each is so different from the others as to be unrecognizable. Players make different choices and DMs have to extemporaneously create the basis for strange twists. The only things which were the same in all three settings was the "Gates of Barovia" scene where they entered the realm. Use the maps, use the ideas, and then create something new from it. However, if what you want is small plug-ins in a complete game of your own making, addons may be the way to go. I use addons in any of my games as a filler when PCs make that choice I was totally unprepared for and I have to give them something while they are headed there. I always have a couple of them prepped for just such an emergency.