Alright, I said I'd do this so now to it. Due to the length, I'm making this a 2-parter.
At 19 points each they are equal to the cost of regular Initiates plus a special skill. This isn't so bad until you realize that regular Initiates are scoring, these guys aren't (a problem due to this being a 4th edition codex in a 5th edition game). So how can you make them worth taking? This becomes difficult when our other Elite options are Terminators and Dreadnoughts.
Making Sword Brethren into cheaper Terminators is the wrong decision, I checked. But let's look at upgrades. For upgrades they can take a variety of things, essentially the same selection than Crusader Squads but doesn't have the doubling option that Command Squads do.
Firstly they can use bolters, or bolt pistols and close combat weapons. Not a mix, though, and it seems like using bolters is laughing at their name. But this choice comes down to what kind of army you are running and how you plan to use them.
Next they may be given Terminator Honors. While this can be advantageous, you must be careful as the price these guys begin to mount up. For a 3++ save that is only another 10 points each you can have some dangerous guys. 39 point guys. Then we need to start looking to see if it isn't better to just take Terminators, which are much better for only a few more points. But you don't have to give them all Terminator Honors, nor all of those ones Storm Shields. A mix is best, and provides some good Invulnerable saves for facing power weapons.
Don't bother with Combat Shields. Compared to Storm Shields you gain 1 attack and 5 points each, you lose 50% greater chance of surviving Power Weapons. Pre-FAQ the option was viable to select instead of Storm Shields since you save the same amount but lose only 16% chance.
Frag and Krak Grenades are better selections than they are for Crusader Squads since if you are charging into cover the improved initiative is much more important for a more expensive unit and with 5 members the Krak Grenades are more reliable against light AV than if they have a Powerfist. Still, they are not staple upgrades to take.
The options are essentially the same as Crusader Squads with a few differences: Power Weapons are now in the second paragraph and costs 5 more points, a Pair of Lightning Claws are available, the Plasma Gun is 4 more points, and you don't get Bolt Pistols with the Powerfist or Power Weapon.
This gives them an advantage in CC over Crusaders since they can take two weapons that ignore armor (One of which can be Lightning Claws). Add in some Terminator Honors for those who have the Special Weapons and you have a deadly Close Combat unit, but we'll get back to that in a moment.
Crusader Seals and Transports are the same as Crusader Squads. The only two changes, tactically, is that Land Raider Crusaders are a nicer option since they help the Close Combat orientation that these guys will have quite nicely, same thing with the Crusader Seals.
And now we get to the skills: I'm going to go in a reverse order from how they are listed. Infiltrate has some nice uses, especially for a mechanized rushing list. You can start the game much closer than the rest of your army and on foot so you free up a few points from not having a vehicle. But you can't have an Independent Character joined and you are on foot. So if you want to have the unlikely chance to charge first turn then this is for you, but I recommend not doing so.
Counter Attack has some different uses, and is best when used with bolters and heavy weapons. This is an option, but you are paying for a Crusader Squad with a special skill that helps if you get charged. But it is only useful if you to get charged and it is always better for you to charge and deny the enemy an extra attack. And you aren't Scoring.
Last is Furious Charge. Unless you are going to be shooting with these guys then this is the skill to take. The +1 initiative saves lives and the +1 strength means your Power Weapons and Lightning Claws are going to wound more often. This is by far the best one as it capitalizes on the advantage Sword Brethren have over Crusader Squads.
So that's all the options and choices you can select for Sword Brethren, so what's the best way to use them? That will be answered in Part 2.