The former will set a garbage alpha value, the latter works. More generally, the former requires much more care and attention to get the types right, whereas the latter just requires that the cast match the method declaration.
If the method signature changes (unlikely for a system header, but possible if you're calling into your own code) then you're hosed. Or you could just mess up and copy/paste the wrong signature, or forget to change it.
Basically, it's pretty error-prone no matter what you do. Using Objective-C directly is much safer (and luckily you usually can, since ObjC is pretty much a drop-in replacement for C).
I totally agree on all counts here. The cast saves you from this one specific thing but there's plenty more to go wrong. I think some people may overestimate the difficulty of adding some ObjC wrappers to a plain C project.
