I think Bourdais would be a good choice. I mean You say compare him to Vettel? Well I think Vettel is Probably the most talented driver in F1, Hes at least as good as anyone, and Bourdais was Very close to his pace, He made q3 in the last 3 or 4 races as well, In a STR thats pretty good driving, Plus hell have 2 years exp to help the team a bit. For the other driver, Charlie Kimball is very talented and fast, but I dont know if hes had enough training to step up to f1 in a year.
And if they were gonna grab A guy for publicity I think Rahal is a lot better then Andretti. I think he actually has some talent.
Marco Andretti, Is a typical open wheel American race car driver. Little talent, lots of money, and training. We have tons of Really talented drivers over here, its just no one over here is willing to help the talented guys make it.