In reply to Removed User:
> Where the rope exits the lower qd karabiner it is 6cm lower than a 12cm one, so you will end up falling only 6cm further.
No, it would be 12cm if you were above the draw, and no change if you were below it. Remember you fall twice the distance from your last gear, so if that distance is 6cm bigger then the fall distance is 12cm bigger. Not that it really matters in this scenario.
> In fact if you wanted to be really precise, then you'd actually have less rope stretch due to there being 6cm less rope being fed out. So you would fall even less.
The quickdraw makes no difference whatsoever to the amount of rope out if you're above the draw.
To the OP: You'll be fine with 18cm draws and they'll be better for trad too if you ever want to do any.