No worries - just to give you a small hint: M&M is not known for very generous mileage earnings, unlike UA...

Low econ tickets on any *A carrier will most certainly end of with 25% on M&M while there is a good chance that UA will cedit 100%.

As said, look at the program earning charts for the airline you fly most and decide. I think you might want to look at UA. Even though it will be hard for you to get status they will most certainly credit 100% for AC on intercontinental flights in cheap booking classes. See below three program comparisons for AC operated flights:

AC - 50% for cheap AC IC eco:
http://www.aircanada.com/en/aeroplan...milesChart.pdf

UA - 100% for cheap AC IC eco:
https://www.united.com/cms/en-US/mar...spx?ItemId=313

LH - 25% for cheap AC IC eco:
http://www.miles-and-more.com/online...l=en&cid=18002