Unless you're doing 40km+ bike legs (e.g. Oly and up), a saddle bag is not going to have any significant drag effect.
That said, I use the
XLab FlatWing.
I carry a spare tube, 2 CO2, wrenches and 2 cages depending on distance and hydration needs.
I use the FlatWing only for races. A lot of people have nightmare stories about it coming off or failing in some fashion, though.
I don't use it for training, just racing, and it's never given me any problems.