When you're choosing which 3 Sectors to use in a particular game, how do people typically make that decision? Are the special effects of each Sector different enough that people will learn to favor some over others? Or are the special powers not different enough from each other to influence the decision so people just pick the ones with names they like the most? Or do they just grab 3 at random?
If the differences between them are significant, how does the game avoid some being obviously better than others so that the same 3 should really be chosen for every game?
The differences are significant. While it is not similar to SotM, the comparison would work. Sectors can be compared to environments (although you will always know the effect). Some are easier to deal with, while others may determine (and undermine) your strategy. One important thing to note is that whatever the effect is, it applies to both the Strike Force ships and the Opposition ship (if it can). So for Oothagos Expanse, it is a great place to stay to buff your own ship, but it will also make any Opposition ships in there harder to kill.
EDIT: To answer you question, yes you could pick "easier" sectors, but you can also always pick Megalopolis.
we just randomize them, because they have one effect and only one effect (well 2 with the Overrun side) the entire game, and as such you may prefer one or another, but for fun sake we just throw out what ever comes up in the shuffle
Even in sentinels after you play a enviroment you know its difficulty and could choose not to ever play it again cause its too hard or play it all the time cause its easy. I always do random and will prolly do the same in this game once i understand what each sector does. Or i guess since there are 3 sectors you could pick one easy one and 2 harder ones or any combo you want right? If so that is pretty cool