@Saturn
I think what puts Shulk over the edge into playable territory is the fact that Xenoblade has the potential to become a big thing for Nintendo with the upcoming "X", regardless of whether or not Shulk stars in the game.
At this point (imo) Golden Sun hasn't been on the rise lately, and I don't really think that Golden Sun is going to get too much more than it did in Brawl. Isaac being an Assist Trophy was spot on, but considering the series hasn't generated much else since...
Yeah, they had a third installment, but there has literally been no news on another installment to the series.
I might be a bit biased because I've never played the series, but I just think that Shulk was popular and requested at the right time, is new and from a series that has had no representation otherwise in Smash Bros., and would absolutely spark interest in a series that Sakurai undoubtedly knows about.
Here's how I break it down:
Both have popularity
Both have absolute potential for unique movesets
Aside from this however, there is the fact that if we take Sakurai believing that he only thought Pikmin and Animal Crossing needed a character around Brawl's time, Isaac may be overlooked yet again. Surely Sakurai will review all of his options, as I'm sure any smart developer would do. I just have my doubts with Isaac.
It's Shulk for me.