What are your thoughts on getting a blue heeler vs. red heeler? Are there temperament issues with either? Is one better in some situations? What about a mix of the two breeds?
The only difference is the color of their coat. If you wanted to breed a red and a blue it wouldn't be a crossbreed as they are the same breed. You might end up with a purple dog though.
As Marcelo notes, the only difference is the color of the dog's fur. They are all Australian Cattle Dogs (or blue heelers). Blue heelers come in both "blue" and red coats. They were developed to help herd cattle, so they are usually great dogs to have on a farm.