Here's an argument for you platnum: Mog has nothing going for him.
Because he doesn't lug a giant sword around? How biased you are.
Mog:
+Was on Nintendo System
+Would be only Spear user in Brawl
+Would represent not only FFVI but Moogles in general
+Rare Dance User
+Probably cost less than Cloud
-Popularity is not nearly as high as Cloud's.
-Would not be the fan favorite
Lets look at Cloud
Cloud:
+Was on Nintendo System
+Would represent FFVII and all of its spin offs well enough
+Has a large fan base
+Is lead of FFVII
-Would be one of several sword users in Brawl
-Would be mighty expensive I'm guessing
- Mightest and most popular Limit Break happens to be similar to Ike and Link Final Smashes
- Next best Limit Break (Meteor Rain) is similar to PK Starstorm
Ok Cloud's strongest pluses are his large fan base (which won't make much of a difference since Brawl will sell extremely well anyway) and the fact he is the lead of FFVII.
Mog would obviously be a more unique addition to Brawl, but he lacks the popularity Cloud does.
That said, if I were Sakurai, I would skip Cloud and go for a Black Mage, Chocobo, or Mog.
Cloud actually seems like the less favorable option to me when you strip away his popularity edge. Popularity isn't the end all be all despite what people say about third party characters. When Sakurai says popularity isn't the only determinant in selecting a character, I'll have to assume that same rule applies to third parties as well since those who say otherwise are pretty much going with what
they think to be the logical decision.
Sakurai, a guy who knows this game is going to sell may be looking for a third party character that would bring a unique flavor to the game. Sonic and Snake, though they may be very popular, have done this. Cloud is a sword user (lets face it), whos greastest techniques already exist in the form of an Ike/Link and Lucas Final Smash.