The term "B-Spline" can be a little confusing in AE-world. I like to call then "NURBS" (Non-Rational B-Spline curves), since that's where I first came across the stuff, in Maya-land.
AE doesn't have anything like that; it uses Bezier curves for everything (hence the confusion, as Bezier also begins with a B).
Mocha does have something similar to B-Splines, but it calles them "X-Splines" (it also has Bezier curves). If you prefer NURBS for rotoscoping, then MochaAE is the way to go. (Note that MochaAE comes with AE.)
Personally, I prefer to use NURBS-style rotoscoping for less-precise work, and Bezier curves for super-detailed stuff.