As Juku121 said.
Cult arms dealers are essentially useless, because by the time you could get them, you already have superios Black Ops and other equipment.
Also, with Sonic tech I usually look at the monetary situation I'm at when I get it. If I need 5M really badly at that point, I'll return it. But otherwise I will keep it. Sonic (mini)shotgun is really useful, especially in underwater missions, because it will almost always guarantee a single hit kill and thus avoid reaction shots from aquatoids. The downside is the ammo consumes a significant portion of Zrbite. It can be useful overground as well, but as said, you may need to be careful about ammo consumption.
Wrt. MAGMA shares:
- Alien gardens tech gives you very powerful chem clips for pulse weapons. Those can be really deadly especially against less armored foes. But by the point you get this, you are usually very deadly anyway. So it's a matter of taste whether you want chem clips or not.
- Terramite and Elerium battery (you need to share both) give you power armor. It is great for underwater and useful on the ground as well. If you already have good armors, this is optional. You can also churn out undersea missions to get a number of armors off Gillman Hero. So while this is not mandatory, this can be rather useful especially for underwater missions if you get it early enough. (For ground missions, by the time you get to this point, you will likely have stolen off at least Power Suits from heavy MIBs so this is not essential, but Power Suits can no longer be used underwater, so..)
- Elerium explosives gives you anti-E115 grenades for TAC launcher. I've never found this all that useful. I suppose this could mainly be used for chryssalid terror missions.
- Tritanium ammo gives you access to a couple of useless guns and allows purchasing tritanium ammo for pulse weapons and heavy shotgun. I've never shared this myself, but if you are at a stage you find those useful, no problem I suppose.
By the point you'd share these, your monthly score would probably already be in the 5000+ range. So the point penalty is not really a big deal. If you share anything, you will also get "slander missions", which you cannot do (you'll always be late) and get additional hit. There are also other similar mission triggers, like sharing tritanium ammo leading to an osiron mission.