You can't deny that more people are aware of who Cristiano Ronaldo is than Nelly Furtado. Probably Kátia wouldn't go to ESC using that old "Ronalda" moniker but for sure she'd use the fact of being sister of who she is to her advantage. And obviously the media would pick on the story making references over and over again to the fact that "the sister of Cristiano is representing Portugal at ESC".
Also I believe it'd be less risky sending Kátia than Nelly. Nelly has had worldwide reputation for over a decade and if she flopped at ESC that would be a bad blow for her. I think it's too ambitious of a move. Sort of like "let's play our last and ultimate card" - this alone would generate detractors from the moment her selection would be announced. So for her own sake I'd rather not see her going to ESC. With Kátia there would be nothing to lose on the other hand
The key thing in here is that a strong song is needed. Nelly, as a songwriter would come up with something more easily than whichever team of songwriters Kátia would hire (or Ronaldo himself for that matter).
I believe that ESC is to non-established artists. So from this point of view I'd much rather see Kátia's debutant attempt in front of a big audience than build result hopes solely based on Nelly Furtado's reputation alone.
What I really think will happen anyway - is that RTP will revamp FdC (minor changes, barely noticeable, ofc) and that some other anonymous artist will represent us in Ukraine without much success. RTP doesn't have the audacity to risk and change. They have not had it since the 50's and I very much doubt that it would be now that they would find their "guts" anyway.
