I would prefer to use 6mm dominos to the 5mm for my cabinetry work. Why doesn't Festool make 6x30 dominos?! I know I can just use the 6x40's but having to switch back and forth between the depth of cut gets annoying, and invariably, I would forget when cutting into the back of a faceframe and just go right through. Not that I would already have first hand experiance with doing that. Nope. 
I just like the 6mm better because the registration pins work easier. With the 5mm, I sometimes end up fighting with the alignment, or the pin catches before it hits the edge of the mortise, or the pin gets caught as I'm moving the domino for the next cut and pulls out, spring and all. Its not like all that happens all the time, but its enough to bug me.
And yes, I know I could make my own, but I have better things to spend my time doing than cutting hundreds of 30mm dominos.
I just like the 6mm better because the registration pins work easier. With the 5mm, I sometimes end up fighting with the alignment, or the pin catches before it hits the edge of the mortise, or the pin gets caught as I'm moving the domino for the next cut and pulls out, spring and all. Its not like all that happens all the time, but its enough to bug me.
And yes, I know I could make my own, but I have better things to spend my time doing than cutting hundreds of 30mm dominos.
