I read somewhere that the Rugulation Doublet is a better option for larger men, as the flaps help to hide the waistline somewhat. Also if your going to really dress up with a plaid and dirk etc. the Regulation Doublet would look better (in fact Thompson in 'So you're going to wear the kilt' says he hates to see a plaid being worn with a Prince Charlie).
I'm hoping to get myself a Regulation Doublet in the new year as I'm a larger man, prefer gaunlet style cuffs to the straight buttons and want to really go to town on the plaid, dirk, long hair sporran etc, etc,