Both his Argentum and Aurum lines basically give you fairly free reign with regards to the design ideas that you have. In other words, you can MTO your own shoes. So if it is a particular design that catches your eye, you can most probably just request for that design, made to Argentum specifications. There may be some designs that are Aurum specific, I suspect, like perhpas, his mind-blowing wholecut tacticus design, but I cannot be sure.
The key differences between the 2 lines would be:
1) Aurum shoes have Aurum specific lasts that are not available for his Argentum line. That being said, I chose a non-Aurum last for my pair. You can do this too.
2) Certain detailing aspects / finer handwork (that require a lot of time), eg shaping of the waist, heel work, finishing, etc, that are done better, on his Aurum line. These things may not be immediately apparent, to be honest, unless you really looked for them.
What I'm trying to say is, his work on the Argentum line is already extremely commendable. And most certainly, far exceeds the work on makers like Edward Green, Gaziano&Girling, John Lobb. ON TOP OF THAT, they are handwelted.
3) His Aurum line shoes have hand stitched outsoles.
4) I'm pretty sure he makes an effort to use better (slightly) uppers for his Aurum line shoes.