My rule of thumb is that each increase in armor tier is roughly equivalent to an level upgrade. In this case the two armors give about the same amount of protection.
Things that might tip the balance:
Are you planning on upgrading this armor, or waiting for scale/plate? If going to upgrade, keep the mail - better scaling with upgrades.
Are you planning on inscribing the armor? If so, keep the leather - iirc the enchantment activates more frequently/more strongly with higher level.
My rule of thumb is that each increase in armor tier is roughly equivalent to an level upgrade. In this case the two armors give about the same amount of protection.
Things that might tip the balance: