I think what was meant is that zilean does not offer in team fights is CC skills. But the lack of CC skills he makes up with his ult and his other abilities. Personally I go for AP build. The reason is that zilean's skills scales well with AP. First, it will help you early on since you can have lane dominance with time bombs. The more damage you do early on, the more you will be able to pressure your opponents in your lane. Now with map awareness and Zilean's set of skills it will be hard for you to die even without defense items. This translate to mid game advantage. However, say both teams are equal by mid game. Zilean with AP can still help out in team fights. With around 200-300 AP (200 being easy to acquire by mid game) you can do good damage to your opponent's dps. Even if they focus on Zilean, you can easily and quickly drop a time bomb and ult yourself. At Chrono lvl 2 that is 750+600 or lvl 3 being 1000 + 600 . So while you are dying and getting revived your single bomb will most likely explode doing damage and hopefully your team focused on the enemy's most dangerous champion (their dps). Also, AP allows you to easily revive your own dps at full health.

I really don't see much difference between the two kinds of zilean, but I lean more towards the offensive I guess. Archangels gives you both, Deathfire's Grasp gives you both, and those are the two items I would concentrate on getting for any build. I would also add in a Mejai's if you want damage and you know you can use it, and a haunting guise helps both by giving CDR and MPen.

So... why pick one or the other when you can be dealing 1200 per bomb AND reviving every 20 seconds?

Really though, as an expert in zilean... the only item that a zilean should never EVER EVER EVER be without, is a pair of Mercury Treads. There is absolutely no reason no to get these ever. Period. If you are not stunned, you will not die. What more can I say