There's very little difference in activation of biceps in chins vs pulls. It's pretty much what you're used to that you do the best with.
Since the biceps is a bit short (slightly) in chinups it makes it a bit easier through the first ~90 deg of ROM which makes it feel easier, but that's just because in pullups you're getting a little more brachialis involvement. Both are elbow flexors so no big difference honestly.
If you're looking to up the numbers I would alternative endurance days (X sets to failure, pyramids, ladders, etc) with max effort 3-5 reps of ~3 sets weighted pullups.