Perhaps someone else would be more qualified than me on this, but the way I understand it X7R are fine for bypassing, C0G are more appropriate in say compensation or filtering circuits. The latter are "better" though. The difference is the dielectric. X7R have a higher dielectric constant, which results in more capacitance per volume. Thus, X7R are used where the "better" C0G capacitors cannot fit.
Also, search for NP0, perhaps that will yield more results.