Your code look good, but there is some room for improvements.

Comment your code

Its is a good pratice to comment the behavior that you code should have. There is a good article written by Jeff Atwood called "Code Tells You How, Comments Tell You Why", when you have time you should read it, but the best way that i find to write comments its to write them before the code itself.

So if i had to write your code myself my first step would be something like:

// iterate all properties
    // check if the the first value is defined 
        // ...
    // if the value is not defined then 
        // ...

The hard part its (almost) never writing the code itself but thinking about it, if you have no problems writing these comments you should have no problems writing the code itself. This helps you to make sure that you undertand the problem that you are trying to solve with code, if If you find diffucults in this step you should spend more time thinking about the problem.

Iterators

You are using forEach but seems that you only need to iterate until the first positive result so you may use some, so your function will not need to iterate all the elements, so your function would be something like:

angular.some(model2, function(m2Val, m2Key){
  if((m2Key === m1Key) && (m2Val !== m1Val)){
      console.log(m1Key +" - " + m1Val + "  CHANGED TO  " + m2Key +" - " + m2Val);
      if((m2Val === decline) && !(m1Val)){
          nullDec = true;
      }
      if(((m2Val) && (m2Val !== decline)) && !(m1Val)){
          nullVal = true;
      }
  } 
  return nullDec && nullVal;
})

Conditionals

Inside the if(!m1Val) you have the conditionals (m2Val === decline) && !(m1Val) and (m2Val) && (m2Val !== decline)) && !(m1Val), you don't need to check m1Val again because its was already checked, the same its true to the block of code of your else statment.

Complexity

If you are sure that all the keys in model1 is always exist in model2 you don't need to iterate model2 to get its value.

Instead of the second forEach if the (m2Key === m1Key) you can get the value of the m1Key in the model2 object without the second loop;

This code have O(n^2) complexity:

angular.forEach(model1, function(m1Val, m1Key){
   if(!m1Val){                             
      angular.forEach(model2, function(m2Val, m2Key){
          if((m2Key === m1Key) && (m2Val !== m1Val)){

          } 
      })
   }
}

This one below have O(n), but will need to make sure that all model1 and model2 have the same keys, or add some conditionals to make sure that the behavior is correct.

angular.forEach(model1, function(m1Val, m1Key){
    if (!m1Val) {
        if (m1Val !== model2[m1Key]) {
            // ...
        }
    }           
}

Even without these improvements your code passed all the test cases that i could come up with, so you just need to adjust your style and get deep understanding of the JS api. I can see that you tried to cover all possible cases that is pretty good, you should continue aim for this as much as possible.