I'd go for Gillespie. Vaas has a tendancy to be woefully inconsistent and have absolutely dead series where he can't buy a wicket. Gillespie to a lesser extent also suffers from inconsistency, but his low points are less low, in my opinion, while his high points are equally brilliant. Even when he's not being successful, he is a useful stock bowler who is quite accurate and can buy a wicket with decent pace and sharp movement off the pitch. Vaas when he isn't bowling well tends to get smashed around a bit.
With regard to Gillespie "losing his edge", I think it's a bit early to judge that. he didn't have a great Australian summer but he was certainly passable, and it was only 8 months ago in India that he was Australia's best bowler by a considerable margin, with 20 wickets @ 16 and a major role in winning the decisive third test. People are mostly judging him on his poor start to the Ashes tour, but I think he will return to form. Vaas at his best is an extremely good bowler, but he's never been able to maintain that for extended periods, and with his lack of pace tends to rely a bit too much on helpful conditions, while Gillespie is an effective bowler on any wicket.
