I noticed that some of Zerg's strategies involve spreading out Overlords onto the map or leaving a zergling here and there at the watch towers or potential expansions. For Protoss, I noticed they'll use perma-cloaked Observers. For Terran, outside of Command-Center Scans, what are your preferred methods for sustainable Map-Control? And, do you have preferences for which units to use based on whether you're playing versus Protoss or Zerg?
I think you're confusing vision and map control. Overlords/observers are great for vision - but they can't "control" a territory because controlling a territory means the enemy is reluctant to go there because it's well protected. For vision, Terrans have scanner sweep - for more lasting vision they have marines and sensor towers. For map control, tanks + marines work very well.
To be honest, as far as map vision is concerned terran are slightly gimped. There is no true jack-of-all trades observer-like unit for terran and we cannot make do with overlords the way zerg can early on What I have done is use the scanner sweep (at a point in the game where I feel that a glimpse into their base gives me enough information) to see if they are going for invisible units. If they are going invisible then i make a raven or two (expensive but necessary) and i use those to look around the map, but the downside is that i have to be extremely careful since they arent invisible. If there are no invisible units then a banshee and marines controlling naga towers works well enough.
Lots of Orbital commands helps with scans, but having air control is huge so you can shutdown the other players map vision. Vikings can run around and take out overlords, and ravens can snipe off observers with auto turrets when they find them. It can be useful to place marines around the map at different expansions and what not to make sure nothing weird is going on. You can also build supply depots at key places on the map too to give you vision if you're that concerned about it.
Usually I just stick with sensor towers and the watch towers. But I also find that leaving my original SCV scout hidden in their base until I want to scout with it later helps a lot.
