With Sebastien Vettel leaving Torro Rosso for their bigger brother in 2009, Torro Rosso's most important issue is how to stabilise the improvement in the team from 2009 onwards.
Four drivers appear to be lined up for this role which begs the question whether Torro Rosso are looking for experience but also talent to grace their line up.
Probably my pick if I was choosing but in a way the least likely, Sato is a proven F1 driver with an impressive record in both lower formula and F1. He also has the experience of building a team around him and creating a strong relationship with a team (Honda and Super Aguri).
Also Sato is quick if a little erratic, although Super Aguri seemed to iron out his erratic nature. In a way he finally become a veteran rather than being the newbie. However bringing Sato into the team would be a major step for Torro Rosso.
Firstly he's not a red bull backed driver and bar Bourdais, Torro Rosso has mainly stayed as the top end of the Red Bull development programme. Bringing in Sato would be a major shift in perspective for the team.
However that is important for Torro Rosso because of the rule changes in 2010, Torro Rosso will have to become a fully independent outfit. Sato would be an excellent choice as he can help develop Torro Rosso just as he helped motivate the Super Aguri team. In essence Torro Rosso would have to change their plans for the future dramatically to incorporate Sato, but I think that they would be foolish not to.
Buemi is probably the exact opposite to choosing Sato as he has no experience of competative F1 and has no knowledge of building a team around him. He is a product of the Red Bull development programme that means that he is fast, but not fully shaped into a F1 driver (look at Scott Speed, and Vitantonio Liuzzi).
Essentially he is talented and highly rated but would not be the best choice for a team that will need leadership in the next couple of seasons.
However he does have one card, Buemi has driven F1 cars quite extensively in testing, indeed he knows the Red Bull car pretty well and has been one of their primary testers, surely a sign that Red Bull do want to sign him.
Buemi would be more of a risk than Sato or Bourdais, but he does have the necessary talent to become a decent racer, he just needs some time.
Sebastien Bourdais: Bourdais has so far been erratic this season. With his excellent background in the US many (including myself) expected Seb to really hit it off in F1. However so far he has only showed glimmers this season: Australia, Europe and Belgium were all decent drives but apart from that he hasn't been that impressive. However Bourdais does deserve another chance.
Everybody knows he's got talent but can he deliver in F1?
This could be helped by having Sato as a teammate. Not a driver who would threaten Bourdais' position in the team unlike a Vettel-esque teammate but also would help Bourdais to realise his full potential. If he was chosen alongside Senna and Buemi he would be in danger of being beaten by a much younger, less experienced driver again.
Mere speculation but it is claimed that Senna is looking all around the F1 paddock, and seeing as Torro Rosso have a seat (or maybe two) Senna must surely be in the frame for Torro Rosso. Like Buemi he has huge talent but doesn't have a huge amount of experience in F1.
He has tested for Torro Rosso but not on the level of Buemi and thus would go into 2009 with the least experience of the four contenders. However he has had quite a bit of success in the lower formula.
Although maybe it would be better if he stayed in GP2 for 2009 and win the championship, just to make himself more attractive to the top teams. Senna, with the least experience of the four, would probably be the most risky option for the team.
So my evaluation? I would personally choose Sato and Bourdais as the best duo. This would however be the safe option as neither look like they are going to become potential superstars like Vettel, although they would most likely cement Torro Rosso in the midfield than a team of rookies.
In which case STR would probably go for Senna sitting alongside Sato, as they look to find security in their development and future but also look to discover new talent.