GG, that study suggests you'd need 500 mg daily for the effect. While that may be fine if you want to reverse existing Alzheimer's, I don't think I'd want to do that without having it in the first place.
There's this as well:
BBC NEWS | Health | Green tea compound Alzheimer hope
The difference here is prevention. The problem though, is drinking the tea isn't enough. You'd need a concentrated amount of the tea's EGCG, which could be obtained with supplementation.
My point here is that drinking coffee does have benefits. It can even prevent Parkinson's apparently. But so can green tea. Green tea doesn't have the same problem of caffeine that coffee does.
The caffeine in green tea is more tolerable due to its makeup, and the caffeine level is much lower to begin with. Also, the carbon dioxide decaf process of green tea is ideal to remove most of the caffeine while maintaining the same levels of antioxidants and other goodness.