Man the problem is that both of them can serve position 1 very good which in this case naruto sosp or minato ..they will both combo and they will do whatever you need as a 1st support ninja (position 2).

this is what books say but if you are having minato or edo diedara as position 1, then get 5k madara and if you have naruto sosp get edo itachi.

Other thing you need to put in your consideration is the 3rd ninja you are having. for example having susano kakashi in your recruited ninjas that means you do not need edo itachi right now and he could wait.

finally, for my opinion go for 5k madara especially if you are faster than most of your bracket. Edo itachi most benifit is to make you have a fighting chance against AGK and Edo heruzin teams. but 5k madara is more tactical with interrupting mysteries and repulse chase and stealing chakra which can be very helpful.

As i told you before 5k madara is a must have ninja for now... edo itachi is made to counter kakuzo and edo Hiruzen teams... so if you have naruto sosp and you dont meet much of edo H teams then go for madara.