What is the difference between a __weak and a __block reference?

From the docs about __block

__block variables live in storage that is shared between the lexical scope of the variable and all blocks and block copies declared or created within the variable’s lexical scope. Thus, the storage will survive the destruction of the stack frame if any copies of the blocks declared within the frame survive beyond the end of the frame (for example, by being enqueued somewhere for later execution). Multiple blocks in a given lexical scope can simultaneously use a shared variable.

From the docs about __weak

__weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.

So they are technically different things. __block is to stop your variable being copied from your external scope into your block scope. __weak is a self delimiting weak pointer.

Note I said technically, because for your case they will do (almost) the same thing. The only difference is if you are using ARC or not. If your project uses ARC and is only for iOS4.3 and above, use __weak. It ensures the reference is set to nil if the global scope reference is releases somehow. If your project doesn't use ARC or is for older OS versions, use __block.

There is a subtle difference here, make sure you understand it.

EDIT: Another piece to the puzzle is __unsafe_unretained. This modifier is almost the same as __weak but for pre 4.3 runtime environments. HOWEVER, it is not set to nil and can leave you with hanging pointers.

In manual reference counting mode, __block id x; has the effect of not retaining x. In ARC mode, __block id x; defaults to retaining x (just like all other values). To get the manual reference counting mode behavior under ARC, you could use __unsafe_unretained __block id x;. As the name __unsafe_unretained implies, however, having a non-retained variable is dangerous (because it can dangle) and is therefore discouraged. Two better options are to either use __weak (if you don’t need to support iOS 4 or OS X v10.6), or set the __block value to nil to break the retain cycle.

apple docs