Well - at the end of the day, the axles typically don't break from a ludicrous about of power, they brake from weird ass angles while applying power, big bumps, etc.
So as long as you treat the stock ones well, the should hold up for a good long time.
Take the used ones - no reason to suspect they are abused or anything. Grab this one - 7k miles
http://www.ebay.com/itm/Scion-FR-S-S...25.m3641.l6368
I bought my used engine from LKQ and it was in great shape.