Both?
For Combat (Specifically, stacking massacre missions), the key is to pick a system where there's only one anarchy faction within 10 Ly. That guarantees all massacres and assassinations will target that faction, ensuring optimal stacking. It does help if multiple systems match that same one, but only if there's different factions.
Then it's just "avoid conflict states" for the factions you pick up missions from, but even if two factions get into a fight, there's five others to choose from still.
Meanwhile, for Trade, while it's much less of an issue these days with the metal price change... just go to any extraction and be allied with anyone, and there's bound to be enough deliveries to fill your cutter, it helps stackability if, like with MMs and Assassinations, there's only one possible import destination. It's why Sothis/Ceos works(ed?) so well, because it's a pair of systems with mutual imports, and no other eligible systems in range. There's a lot of misinformation about the Boom state offering "better rewards"...
there's nothing to suggest boom state is a factor in mission rewards. Others say you get missions with "higher paid goods"... I've found no difference between getting missions to deliver, say, Gold, in either of a None, Boom or Investment state... I have a few agri systems nearby, and I'm just as likely to get "Deliver Biowaste" as I am "Deliver Palladium".
The only factors that affect payout on deliveries are, in order of importance, Value of Cargo, Distance to destination in system, Mission Rank, Distance to target system in LY, Volume.
Maybe negative states like Bust and Famine would affect the missions, but since this is Elite: Best Friends, the threat of a negative state causing you issues, especially if you're farming this stuff, is negligible.
There's anecdotal evidence that corporate/cooperatives offer more delivery missions, for <reasons>, but again, that's just a factor about picking your location.... likewise these days you obviously want extraction with something like Industry or High Tech nearby which consumes these goods, and preferably only one system within (20Ly?) range like that... but all these are fixed, static considerations.