While I think both are good bowlers, neither have achieved their full potential yet.
Amir is about 2 years younger than Starc and I believe will end up being a great bowler and legend by the end of his career.
Starc has the better stats right now and if he continues to do well, he could be a legend, but right now he is too expensive.
I tend to focus on Test Cricket, as I think the shorter formats are unfair to bowlers.
I thin to be a truly great bowler your strike rate in test cricket needs to be around 45 and your average around 23 to 27.
Both of these guys have a ways to go to get there.
Just based on the test stats I see, Starc is slightly ahead of Amir, but I would take either bowler in my side.