This is not incremental vs differential really. Basically Crashplan always does incremental backups and when you restore it can create any point in time from that data.

The idea of incremental and differential back ups really applies to when you are using storage like tape or CD where a tape could be lost or destroyed.

Differential has everything since the last full backup so with a full backup gives you everything for that point in time since the last full backup and doesn't rely on intermediate backups.

Incremental only stores what changed since the last backup of any type i.e. you need multiple tapes/CDs to capture a point in time since the last full backup.