I've been playing AoS a lot these days, and been to a few events. I'll try and answer your questions as best I can.
So, out of all of those, I have to strongly discourage the Swifthawks Agents. Great models, but they are fairly terrible. You'll win some games, but it'll be a struggle. Also, most of the models in the range you can't buy except in the discontinued 'Spire of Dawn' box. There are worse factions to play, but not many.
Also, the Swifthawks lore is that they are literally the mailmen of the mortal realms, kinda lame.
Of the others, I think all have merit. Keep in mind, AoS is a melee game, so pretty much all factions are combat-focused. Armies which rely on shooting really struggle right now.
The Nighthaunt
They've got a spiffy new book and spiffy new models! An army of ghoooosts! Boo!
Pros: I love the models and I think the fluff is cool. They are current and very good.
Cons: They might be a bit too good (?)--like, are they just gonna roll right over the Overlords? Kinda seems like they might. Also, dwarves Vs ghosts is just a...weird matchup.
Nighthaunt are good, but I think they are good because they are a meta-busting army. Most folks focus are taking units with lots of rend, for dealing with armies like Stormcast. Nighthaunts don't give a beslubber about rend. The best way to kill Nighthaunts is with horde-killing weapons, which folks often don't take.
They do have fantastic models, and a lot of options. I think they are best as a Horde army, but at least they are an easy to paint horde army.
Beasts of Chaos
Granted, the new book isn't out yet, but I've always like the idea of these guys.
Pros: Sky-dwarves raiding beast-infested wastes is a cool idea. Love the Gors as models.
Cons: They seem kinda one-note as far as gameplay is concerned--mob up and charge really fast. I'm not crazy about a lot of models in the range, either. Also the prospect of collecting so large a horde army...shudder...
Keep in mind, this book will also include Monsters of Chaos, Warherd, and Thunderscorn, along with the Brayherd. So, there could be potentially multiple army builds. But, as the only wizard in the faction is in Brayherd, you'll still want some of that.
Hard to say anything though, as the book isn't out yet. This is the first multi-faction book, so I have no idea how it'll work.
Slaves to Darkness
Chaos Warriors! One of the best kits ever!
Pros: I dig the models a lot, the theme works out (dwarves raiding chaos strongholds for ur-gold), and they seem pretty strong.
Cons: Also a one-note playstyle? Run and kill them with axes, right? Am I gonna do something besides charge straight at the bad guys?
I think this army has a lot of great options. It has fast objective grabbers/distraction units in Chariots/Maurader Horsemen, Chaos Warriors are one of the most durable battleline in the game, and their heros are tough and great support. Sorcerer Lords are arguably the best wizard in chaos. They have a few interesting battalions in the Chaos book. Their allegence abilities are sort of meh, as well as the artifacts, but at least the Realm artifacts mitigate that a bit.
Marauders, although terribly models, are one of the best horde units in the game. Also, the army can be easily played as Maggotkin of Nurgle, Disciples of Tzeentch, or Blades of Khorne if you want to mix things up.
Ironjawz
Elite, armored orks? Weird.
Pros: Dwarves Vs Orks always plays well, they seem pretty strong, and they have a bunch of options.
Cons: Playstyle seems weirdly similar to 40k orks. The paintscheme GW uses for the models is eye-gougingly terrible so I can't tell if I like the basic models or not. They seem very slow.
This is the only army you chose with literally no ranged attacks. However, they are extreamly fast. It's not hard to get +4 to charge distances with this army. 'Ard boys are very tough models, but the army is fairly slow otherwise.
The models are great, just check out google image searches. They have a great wizard too, with a fantastic spell, and is also good at casting endless spells (like the Chromatic Cogs, which you will want).
Probably best played as a horde army. Swamp the board, charge quickly, and outscore your opponent on VP's before they can fight out of their deployment zone.
If you want to compete, Anything but Swifthawks can do well. If I were thinking of a 5 round event, I think Nighthaunts could win 4-5 games, and slaves, and Ironjaws could win 2-3. If you're experianced with the faction, and play well, you could maybe win 4/5 games. I think that Ironjaws and Slaves to Darkness might struggle agaisnt the top armies in the game. But, Slaves to Darkness have some of the best mortal wound defense in the game.
Swifthawks will not do well at Tourniments. You will rarely win games. To be fair, Khardadron Overlords are probably in a worse place than all the factions you listed, besides Swifthawks. They are struggling hard being a shooting army, in a melee game.
Really, the choice comes to you. It'll be an army you have to build and paint, so I wouldn't base your choice entirely on how well the army can do. Especially as Ironjaws and Slaves will eventually get updated battletomes. I think all of those choices can be fun, and put up good fights. Buying Swifthawks I think will be a waste of money, unless you really want to play them, and try to do as well as you can with a handicap.
I'll happily answer any AoS based question you have.