Tokyo Dome was seated (AND numbered) but that doesn't mean people watched sitting down and everybody went crazy anyway. :twisted:
And trust me, with the wait between the stadium opens and the concert actually starts and the length that X lives are, you'll be happy to had the occasional opportunity to sit down long before the live's over.
Personal recommendation: take umbrellas. Don't know why but X lives' days tend to attract rain... well, you could copy the people in the lines at Tokyo Dome and sing "Endless Rain."
EDIT: Standing at such a large event would be dangerous because of crowd behaviour, for example, everybody pressing forward when people get excited and everybody wants to get close to the action, which could lead to people being crushed and panicking when they are caught too tightly in a crows which in turn could cause a mass panic. At Tokyo Dome, and Ajinomoto Stadium as well, they even had partitions between the blogs, even though it was all seated.
In fact, in the early X days several publicity events of X had to be canceled on the spot because of fans' mass panics and that was at events for a few hundred people only. Can you imagine what might happen in a situation like that with several thousands?
Even sitting, from an emergency crowd control point of view, events like Tokyo Dome are security and logistics nightmares.
And in Japan, most large live houses, like Tokyo ZEPP, have actually barriers/space breakers in place during performances.