Between the VM-2 and the Watch-6 I would opt for the VM-2 because of the re-enforced mouth-band. A re-enforced mouth will help to give a shooter a solid platform on which they can reciprocate the gun on and place the gun in should they only have one arm to use and the situation require a reload or malfunction clearance. If for no other reason a holster that remains open once the pistol is withdrawn is a smart thing in my experience.
There is really no perceivable difference in bulk since the extra band of leather around the mouth adds an unnoticeable amount of thickness once the rig is secured in ones waistband.
That said (and having worn the Sparks VM-2, W-6, S/S, SS2, EX, as well as a host of others from various makers) if you are looking for the best and most concealable IWB rig I would recommend the Alessi PCH above all others.