It's not the just the voice or the songwriting...it's the production of the whole song. Let's see, a 'bad' (we're not talking tone-deaf here) voice + good songwriting and good production = record sales and airplay. A good voice + bad songwriting (again, we're not talking tone-deaf writing) and good production = cut-out bin.
Songwriting wins. (These days, promo dollars help too.) That being said, there are such excellent vocalists out there who also can inflect 'artistic soul' to anything they sing so that it sounds salvageable even if it's bad. I'll use the aforementioned Aguilera as an example (even if I think she's a ho.) Actually, for a combination of both songwriting and voice, I'd look to Norah Jones' and Alicia Keyes' latest. I'd give Evanescence's Amy Lee the voice, but not the songwriting. The voice grabs you at first, but the song wears thin on you after a while.