My answer would be that Eb<Ea (field in the mica capacitor is weaker). The
way of justifying this is as follows. Draw a parallel plate C so that the
left plate is negative. Think about all those dipoles inside mica. How do
they orient in the field? Their negative sides are now pointing to the
right. The net effect is as a negative layer of charges next to the plus
plate and positive layer of charges next to the minus plate. The field
created by these layers weakens the field which would the plates create
in a wacuum.