I would use a soft ferrite bead in any case. Only this one shows proper performance even at the highest frequencies. Just take the one you use already.

100nF can be increased, if the choke does not show relevant series resistance, which is the case for most ferrite beads. Normally, people use a 10µH choke, which shows series resistance of several Ohms. But then a delay time is fabricated, which makes it difficult for AVCC and AVref always being at same potential, for instance on power-on or power-down.
You can add some series resistance, but then check with a scope whether AVCC and AVref are always at same potential (a difference of up to +-0.3V is allowed though).

Kai