I'm not too sure on this one.It's a common word,that's for sure.The only problem is it's not a place,person,product,or service.It's just a common word.
No doubt "which.com" would be an excellent name to have,I just personally think it's way too early for a common word IDN to have significant value.Maybe in the future.
I would stick with names of places,products,services,or otherwise unique IDN's like the currency symbols only because cuál.com doesn't suggest anything as to it's content.