If they want both the WDC+WCC every year like RBR they'll get in Seb or Lewis to partner Alonso. Since Lewis has no real interest in F1 history and Seb had a Red fiat panda? as his first car I think it's safe to know who wants that seat more than anybody.
I guess I can see the lure... If I were a racing driver, I'd want to drive for Ferrari. If you go there and do well, you're immortalized and adored by some of the most passionate fans on the planet. So I can see that side of it.
Vettel is still young and could still be in for a shout of his third straight title with RBR... That would take some walking away from.

. We from down under can choke like no-one else in the world can, especially in motorsport: Webber (2010), Will Power (Indycar 2010, 2011 and perhaps this year as well), and now Mitch Evans, except he actually pulled it off!