As mentioned, they follow you for a time as they did the first time, and only after where it would have asked "Do you want X to join your party" the second disappears. The brief bits of bonus dialogue are as follows:
Double Templar
After releasing and talking with the "Warrior"
Templar: Why have you come here, brother?
Warrior: One of our number has forsaken his vows and joined the demonic coven that infests this fallen cathedral.
Templar: Damn that traitor! We will make him pay.
After re-arming
Templar: Evil cowers at the sight of just one templar. Two will bring it to its knees.
After killing Jondar he simply waits at the top of the steps then disappears.
Double Scoundrel
After meeting the clone:
Scoundrel (1): Wait, aren't you from the guild? How could I forget a handsome devil like you!
Scoundrel (2): Why, yes, of course! It's good to see you again!
He simply leaves when you start his quest at the waypoint.
Double Enchantress
After getting the first quest from her:
[Your] Enchantress: She seems very familiar...
At the roadblock:
Enchantress: You must be one of my sisters! How wonderful it is to see you again.
After the bridge she simply leaves.
It doesn't matter in terms of mechanical difference, all resistances will work the same.
It also doesn't matter in terms of most gear, as elemental resistances fall under "Random Magical property" like everything else, so they all have the same chance to roll on an item.
However, with those things said, physical resistance is probably the worst one to stack. Generally you want items that have both "+All Resistances" AND "+Specific Resistance". The problem is that all other classes (and especially barbarians) also want armour with both "+All Resistances" and "+Physical Resistance" so those armour pieces tend to be more expensive.
Another note is that there's one legendary helm (Andariel's Visage) that some monks like to use due to the Attack Speed Increase (which normally can't be on a helm) which always has poison resist. For this reason, you may want to use poison resist in case you ever want to use Andariel's Visage, but at the same time, most all res + poison res gear is slightly more expensive because more people use poison res.
Out of all the other resistances, they're all about equal, but keep in mind that against certain elite affixes, certain resists are better, so people who have trouble with molten/fire chains might buy some fire resist gear, and people who have trouble with Arcane enchanted might buy some arcane resist gear.
I personally use cold resist due to the fact that it's pretty much only helpful against frozen affix champs (which most people can deal with quite easily, and the orbs don't do too much damage anyway, it's the stun that people have a problem with), so it tends to not be used as much as other resistances.
Best Answer
You rescue Kormac the Templar from cultists in the cathedral during the Act I quest Reign of the Black King.
Lyndon the Scoundrel is met trying to con some artifacts from a woman and her village during the first part of The Broken Blade.
Early in Act II, in the first half of Shadows in the Desert, you meet the Eirena the Enchantress who clears illusions away that hide cultists.
As for the camp followers (Artisans), The Blacksmith, Haedrig Eamon points you to where the crown is in Act I and he joins your crusade after you find it. The Jeweler, Covetous Shen shows up in Act II hiding in a barrel.