I would lean towards the weaker shaft. That way if it's too weak you can always add material to the side plate to make it act stiffer. It's easier to tune a weak arrow than it is to tune a stiff arrow without having to buy more stuff to add up front. At least with a weak arrow you can add more strike plate material or cut the arrow shorter to make it stiffer. Unless of course you wanna have an arrow with EFOC, in which case you would want to start with a shaft slightly on the stiff side. But, that's a whole different ballgame altogether. Until then, I would err on the side of slightly weak and try the 2018s.