Difference between Mentor Boost and Mentor Xtra implants?

Mentor has the new Boost implants available but the marketing information doesn't really describe the difference between them and the Xtra implants, only between Boost and traditional Mentor implants. Both seem to have a higher fill and thinner shell. I am looking to achieve a more full/augmented look. Keeping a natural feel is not as important as I would prefer more firmness anyway.