In a standard furball, do you chose your target based on plane, alt, proximity to you, what?
In a furball, one will have multiple threats, so the decision has to be made and made quickly to figure out which bandit is proposing the most immediate threat, but only limiting your decision to the above 3 choices can be a mistake, the SA mental picture is much more involved and larger than this...
in asking the Above question, what really is in question is how ones mind works when in a multiple opponent vs just you, or possibly you and other friendlies........ so you have to QUICKLY do your visual scanning around you, QUICKLY decide who is where both enemy & friendly, QUICKLY determine who is on who and then QUICKLY decide what to engage first, then 2nd, then 3rd........ it will depend on how much info you can process and how many moves that might or will happen in the future you can forsee taking place....... mental overload is what usually bites one in the 6 and causes them to get shotdown
For me, I tend to leave the -51s alone since they have E and tend to burn up on reentry, and focus on planes with co-E and proximity, with type of plane being last favorable. But I do find myself behind Spits more than anything. That wing root is too easy to hit. I love watching it snap. 
I would myself, personally, not discount any plane regardless if they are below me, co alt or above me, strictly talking in reference to being in a furball environment....... and if a person puts a co-e plane at the top of their list, they are opening themselves up to being drawn into a tussle because you are CO-E, meaning you are equal to or current engaged opponent and you are going to have to make him make a mistake or out fly him, thus causing you to possibly lose more E and become a sitting duck for all the other bandits around you......... because, one most times will become target fixated on the co-e opponent and forget about the first 1 or more mental pictures they took scanning around them once hooked up or locked up in a 1-on1 fight with that closest co-e co-alt opponent, letting slip from their minds that they are in a furball environment
regarding the practice of SA ( Situational Awareness ), it is something one learns the definition of, then starts practicing their SA by the forming of GOOD SA Habits which one never fully masters, it is something one continuely practices to stay on top of their game the best they can as long as they participate in this type of gaming environment!
hope that helps
TC
