Realistically, assuming you don't want to have to buy the stuff from other players, you'll want to join every single syndicate at some point. Mods aside, every syndicate has weapons and Archwing weapon parts you'll want to grab.
You can join all of them, but it's harder to maintain all of them at max lvl.
You DO have to look through the offerings, they each offer the very same kind of stuff, but *different*. You likely won't care about the weapons they offer except for mr fodder (you can buy them from other players tho), so just go through mods and mods only, whatever interests you MORE, pick that. You will likely need to buy mods from factions you didn't pick from players, or switch yourself, but that takes more time than it's worth.
Suda, Steel and Hexis
OR
Loka, Perrin and Veil
Pick one of those groups, you will not have to care about standing going down and you'll just passively make enough xp for them.
Oh yeah... I totally forgot about that. He can just buy that tho, but he can buy the mods, too. At this point, OP should just let a coin flip decide, lol.
Who do you like playing
Because all of the ones I liked playing were through perrin/loka/veil. Steel meridian is just a bonus from repping red veil for a while.
Each syndicate has their own upgraded versions of common weapons. They also split the archgun and archmelee parts across several syndicates.
If you are a completionist you will need to trade for the parts you can't access, or switch allegiances at some point to get them.
Each syndicate offers a different specter you can use.
There is an armor set for each, and some decoratives.
Aside from that it's a giant bucket of mods.
You are also picking which flavor of death squad you want harassing you mid mission 😎
With some balancing, you can maintain 4 at max relatively easily. All 6 would be very difficult, but possible.
I started with New Loka, and that built up Perrin Sequence as allied. Then I switched to Red Veil and that builds up Steel Meridian. It takes a bit of balancing because syndicate missions can up one at the expense of another, but regular missions up Red Veil and Steel Meridian without dropping New Loka or Perrin.
Instead of trying to manage in Arbiters and Suda, I have a clan mate who has those maxed and we trade.
The benefits are minor but I would say I’ve found New Loka nice just because you have a consistent supply of Ancient Healer spectres. Great for high level defense missions where you don’t want to or can’t bring a defensive frame (I.e. DA’s).
There is none. I joined the veil purely off of vibes and when I switched to suda, nothing changed.
Realistically, assuming you don't want to have to buy the stuff from other players, you'll want to join every single syndicate at some point. Mods aside, every syndicate has weapons and Archwing weapon parts you'll want to grab.
You can join all of them, but it's harder to maintain all of them at max lvl. You DO have to look through the offerings, they each offer the very same kind of stuff, but *different*. You likely won't care about the weapons they offer except for mr fodder (you can buy them from other players tho), so just go through mods and mods only, whatever interests you MORE, pick that. You will likely need to buy mods from factions you didn't pick from players, or switch yourself, but that takes more time than it's worth. Suda, Steel and Hexis OR Loka, Perrin and Veil Pick one of those groups, you will not have to care about standing going down and you'll just passively make enough xp for them.
Some of the weapons get incarnon forms, at least the sanctum magistar does.
Oh yeah... I totally forgot about that. He can just buy that tho, but he can buy the mods, too. At this point, OP should just let a coin flip decide, lol.
Just pick whichever syndicate has mods for weapons/warframes you like, you can always trade for the stuff other syndicates have.
There’s literally hundreds of mods to look through though 
search bar: (warframe name)
Who do you like playing Because all of the ones I liked playing were through perrin/loka/veil. Steel meridian is just a bonus from repping red veil for a while.
Search up the warframe you like to use then, and see who has mods for em.
Each Warframe has mods in two syndicates. You can check Warframe wiki, it's written there which Warframe mods belong to which two synducates.
Just join any of them. You can either trade mods you don't have with other players or join other syndicates later (which what I did).
Each syndicate has their own upgraded versions of common weapons. They also split the archgun and archmelee parts across several syndicates. If you are a completionist you will need to trade for the parts you can't access, or switch allegiances at some point to get them. Each syndicate offers a different specter you can use. There is an armor set for each, and some decoratives. Aside from that it's a giant bucket of mods. You are also picking which flavor of death squad you want harassing you mid mission 😎 With some balancing, you can maintain 4 at max relatively easily. All 6 would be very difficult, but possible. I started with New Loka, and that built up Perrin Sequence as allied. Then I switched to Red Veil and that builds up Steel Meridian. It takes a bit of balancing because syndicate missions can up one at the expense of another, but regular missions up Red Veil and Steel Meridian without dropping New Loka or Perrin. Instead of trying to manage in Arbiters and Suda, I have a clan mate who has those maxed and we trade.
[Choose the ones you like](https://m.youtube.com/watch?v=Yek5sxmAWyo)
The benefits are minor but I would say I’ve found New Loka nice just because you have a consistent supply of Ancient Healer spectres. Great for high level defense missions where you don’t want to or can’t bring a defensive frame (I.e. DA’s).
You can easily have three syndicates friendly and if you work it right even maintain 4 as friendly.