Personally, I'm leaning towards Jigglypuff as the long time veteran. However, the Gematsu leak does bring up a rather interesting point: that there was a debate about whether to cut Ness or Lucas. While it seems Lucas is the one who's going to get cut, the fact that there was a debate at all means that Ness, and by extension, the original 12, are not absolutely safe. Taking that into account, Mewtwo getting in over Jigglypuff seems like a very real possibility. After all, Mewtwo essentially has everything going in his favor for joining the roster, aside from the fan made idea that slots determine the roster. Jigglypuff, on the other hand, only has her status as a veteran to rely on. She's really not a prominent or popular pokemon nowadays. So if being part of the original roster is not a free ticket into every future installment, Jigglypuff may end up being cut in favor of a more suitable candidate. That being said, I would still say that Jigglypuff is the more likely of the two, but it could go either way. Ideally, though, we'll see both. There's not much valid reason for why we can't have both.