Have to say I'm one for Mossy on my Late-game tools.
My set usually consists of:
Broadsword: Handle = Alumite Tool Rod, Guard = Paper Wide Guard, Blade = Manyullyn Sword Blade. 6 Modifiers (Netherstar + One Diamond and One Gold Block and the extra modifier supplied by the Paper Wide Guard = 6 Mods.) Full Luck, Mossy, Save 4 modifiers for hammering on with Quartz for Sharpness bringing the Blade up to a staggering 21+ Attack Damage, capable of easily one shotting.
Pickaxe: Handle = Manyullyn Tool Rod, Binding = Paper Binding, PickHead = Manyullyn Pickaxe Head. 6 Modifiers, Full Luck, Mossy, Save 4 for Haste (Redstone.)
Shovel: Handle = Cobalt Tool Rod, ShovHead = Cobalt Shovel Head. 5 Modifiers, Mossy, Save 4 for Haste.
Hatchet: Handle = Cobalt Tool Rod, HatchHead = Cobalt Hatchet Head. 5 Modifiers, Mossy, Save 4 for Haste.
Hammer: Handle = Tough Manyullyn Tool Rod, Plate 1 = Cobalt Large Plate, Plate 2 = Ardite Large Plate, Hammer Head = Paper Hammer Head. 6 Modifiers, Diamond, Silk-touch (Silky gem,) Mossy, Save 3 for Haste.
In all of this, the only thing that would ever catch out my eye would be the Hammer, why a Paper Hammer Head you ask? Because it's easy to repair if Mossy ain't cutting it, Simply smack on down a Tool Station and apply a couple Paper Stacks to the Hammer and you're rolling. The base Durability for the entire Hammer is 10k+ so it's ideal for me entirely. A cheap ore-grinder.
In all honesty though, Flux has it's advantages, I know you can wirelessly charge the tools and this keeps attracting my attention and making me want Flux so much more now... Both are pretty good really, but in the end I'd say:
Flux for:
Easy repair
Wireless charging
Late-game tools
Mossy for:
Free repair O.T. (Over time)
Cheap and easy to acquire
Early-game tools