I am thinking a double chainring. Couldn't imagine anyone using a triple on a bike that they are trying to get under 20lb. I know the weight penalty isn't much but to go for that weight you won't be able to give up anything.
I have noticed that when shifting onto the big rig the chain does not sometimes go on cleanly but will overhang the outside of the teeth for a portion before getting engaged properly. This then interferes with any outboard timing belt/chain if there is minimal clearance. On our bike it sometimes throws off the timing chain. This doesn't happen when dropping down onto the small ring.