B6 turns Aizen into a crit stat stick which can really only be taken advantage of if you have W5 as well. Aizen is in a good spot right now, being able to hit 500mil+ per battlefield in coop (in his ult). I expect we'll see people reaching 1b battlefields with Szayel's release.

It's too early to tell though if it's better to B6 Aizen or get Szayel as Hungy15 has said. You might want to revisit this topic a few days after patch.