Some people think XSI has the best animation tools in the game, I am still undecided
Maya is certainly good, maybe / probably even better for animation... what makes XSI winner IMO is because it's so good and even performer in all key areas - usually apps are very good in some but horrible in other... Like LW is very good modeling tool but the animation tools have always sucked. It's nice if you can do EVERYTHING with one app, both modeling (+texturing) and animation, without need to import / export stuff between different apps.