Does changing EDTA concentrations affect the amount of calcium ions in milk?
Since this post was a while ago, I'm assuming an answer has already been decided on, but I'll take a shot at answering anyway.

If I were you I would rely on concentration changes of EDTA as the independent variable since it allows for more precision in your titration. If you were to vary temperature, it would have to be the temperature of the titrant such that volume changes could be accounted for (since analyte is usually in an approximate volume of water). While volume fluctuations of titrant are theoretically possible, they are rather difficult in a laboratory setting and not exactly worth the effort unless you have lots of time and expensive equipment to control the temperatures exactly. Not only that, but volume changes of water are only approximately 5% from 0 Celcius to 100 Celcius.1 Thus, as I said, changes in [EDTA] would be more efficient and easier to manage.

Thermal Expansion of Water