Hi Anna, I would use SAW in this case because the focus is not so much on the action of watching but on the fact that you have seen it. (Of course the other way is possible too, I will give you some examples for both cases). Examples 1: I saw your ad on TV yesterday. I saw an interesting advertisement for a summer vacation in England. Examples 2: I watched advertisements all day yesterday. I was watching your ad on TV when the earthquake struck.

I think that is correct