I would agree with your general assessment. Another thing to consider is that Lich Bane works very well with him, so if you plan to buy that as well as Morello, you may consider only taking 10% CDR in runes.
As a personal preference, I actually take a full 30% CDR in runes with no AP and buy Archangel's instead. The power curve is different, but I find that I like it better. Comparing just the items, a completed AA Staff will always provide more AP than a Morello. If you include the AP loss from taking CDR runes instead, you need something in the 1600 total mana range for it to even out.
As far as "mana sustain" goes, I prefer the mana refund on cast rather than relying on getting takedowns to restore my mana.
Comparing the other attributes, AA Staff gives up to 600 more mana than Morello and gives an active shield vs greivous wounds under 35% health for 200g less.