When I first started playing OpenXCom, I learned that reaction fire occurred with the weapon with the fastest snap shot, if a soldier had a weapon in each hand. But somewhere in the development of TftD, I think this was changed to the weapon always being the one in the "active hand", which is the last selected hand (and the visible weapon, in case a soldier is using two two-handed weapons).
I am playing through Piratez now, and I have a lot of gals with pistols and swords. This "active weapon" thing is being tricky, where a gal with shoot a pistol at an enemy stepping out of a door in front of her instead of stabbing it, and I am not sure how it works if the active weapon is a sword and an enemy walks more than a tile away. Does the soldier use the pistol despite it not being the active weapon? Does he wait until hopefully the enemy steps within the melee zone?
Overall, I think I would prefer a "use melee if possible, otherwise lowest TU cost snapshot" approach, which is sort of a hybrid between the old and new schemes. An option to decide how reaction is handled (between the two existing schemes, and maybe a "active hand if possible, other hand otherwise" scheme) would be neat.