Mottos are not a compulsory part of a coat of arms, and when granted to you by either the Lord Lyon or the College of arms, its really up to the petitioner if they want one. Arms are in themselves meant to give a 'motto', and the Middleton one hopes to accomplish that.
In reference to the coats of arms above, the labels would be differenced as they always have been according to the rules of heraldry.
1st Son of Sovereign: Plain white label
2nd Son of Sovereign: A crescent, although this mark of cadency can somtimes be catered to the grantee.
The arms would also probably be granted around their 18th birthday, and would not change if they became Dukes.