You could not even need a pop up, you can do this with javascript. basically this is sudo code of how it would look:
if(computer.brand = mac){
download.mac = true;
}else{
download.windows = true;
}
with this code you would eliminate the pop up and therefore limiting the amount of decisions someone would need to make.
believe me, there are people out there that might not know if they have a pc or a mac. it might be 1/1000000 but its still likely
also, users like things handed to them. so the easiest way for them would be less decisions.