Squeak – You are definitely right that HKB”H definitely gives different people different challenges, and each person has a personal tachlis to fulfil during ones lifetime. The people who are born into nonreligious environments are probably stronger than those born into religious families, because Hashem decided that they HAVE the strength to overcome that particular nisayon, whereas a FFB may not have that strength, and therefore HKBH does not put him into the matzav on nisayon which he cannot overcome.
By Moshe and Ahron, the Torah sometimes precedes Moshe and other times precedes Ahron, to show that they were both equal in greatness. Everybody points out that of course, Moshe was the greatest Prophet ever? The answer is that both of them fulfilled THEIR OWN POTENTIAL, although Moshe’s madreigo was higher than Ahrons. So to answer the OP, if both fulfil their tachlis, they are equal in stature.
It also says Lfum Tzarah Agra – that reward is calculated in proportion to effort. Somebody who was handed everything on a silver platter and becomes a Gadol Hador may NOT get as much schar as somebody who works on himself to become frum.
However, I was once told a Mashol of a Dinner where the children were given Hotdogs and Chips, and the adults were served a four course gourmet meal. Each one enjoyed their meal immensely. Giving an adult hotdogs or giving a child a gourmet meal would not have satisfied either of them. Each person received exactly what he needed. And although the lamb chops were BETTER than the Hot Dogs, since a child cannot appreciate it, for him it is pointless, and worth less than the hotdog. It would not be a reward for a child to receive a gourmet dinner, because he cannot comprehend it.
The same thing is with us in Oilom Habo – if one person fulfilled his tachlis to the full, he will get the reward he deserves – but you cannot say that a “pushite yid” will get the SAME reward as a Rosh Yeshiva, but rather in proportion he will. Giving a simple person the same reward as a Rosh Yeshiva – even if he did put in more effort and time and thought to become what he became – is stupidity. (Dunno if it is true, but it is definitely something to think about!)