In this guide, I will assume you are using the bootlegger/Ali Baba's wee booties.
I will go over the stats and mechanics of every sword.

The Eyelander and its reskins (HHH's Headtaker; Nessies Nine Iron) reward you for getting kills with it with speed, max health, and shield bash damage all increased for every head you take.

You will have -25 max health without any heads, so you would have 175 health with the booties and 150 without them.

Heads:

The buffs you get from the Eyelander getting heads has a limit. After 4 heads, your max speed and heath will no longer increase, and at 5, your shield damage wont increase.

Every head you take increases your max health by 15, so it will take 2 heads to make up for the lost max health downside.

With the booties and at least 4 heads, you will be at the same speed as a scout.

Your heads are lost when you die, but if you kill another demo with an eyelander and heads, you will take his head and the heads he has collected. This is the same for snipers with the bazzar bargain.

This weapon makes very little sense to me

Picking up ammo packets will recharge your charge by 100% for big ones, 50% for medium ones, and 20% for small ones.

When you hit an enemy, 20% of your charge is returned to you.

This has a few bugs, but I won't go over them.

If you are using a sticky launcher or grenade launcher, your max ammo will be drastically reduced.

I don't know if this is true, but getting a kill seems to fully replenish your charge

The Half-Zatoichi gives you 50% of your health back every melee kill you get.

if you kill a player and your health is above 50%, the excess health earned is returned to you as overheal.

If you have it equipped and another player using the half-zatoichi hits you with it, you will instantly die. You can do the same to them.

If you switch from this weapon to another and you aren't overhealed(I think) then you will take 50 damage.

The Claidheamh-Mor is actually quite simple.

If you get a melee kill, your charge meter will increase by 50%

Your charge lasts half a second longer

You take 10% more damage from bullets.