I would go with the 908 if it were between the 2 of those.
I have both the Audi R10 and the 908 and the Audi is just a bit weak on power compared to the 908 and some of the other LMP cars as well.
In the Audi line I would go with the Audi R8 but it can only be found in the UCD.
The 787B is a good choice as well not quite as fast as the 908 but will take the R10.