FWIW, I have had good luck using light tubes sized on the small side. They are lighter and make the tires easier to mount. The spares are also less bulky. If you carry two spares that is 4 tubes in total. Since it is 4 of them the weight difference adds up enough to make a difference IMO. The lighter thinner tube means a slightly better ride in part due to having the effect of making the sidewall a bit more flexible. You do have to top them off a bit more often though.

It is best to make the decision based on what works for you for the type of touring you do, but for me I'll stick with thin slightly undersized tubes. They have served me well on quite a few long tours in a wide variety of conditions including a lot of miles in goat head thorn country.