Meet Bors, the A5 merc I hired in
hell difficulty at 91, and leveled to 92:

Now meet Frisian, the A5 merc I hired in
normal difficulty at 91, then leveled to 92:

As you can see, their stats are 100% identical.
I'd read on various D2 merc guides that normal-hired mercs have slightly better stats, and higher level auras. I have no way of proving whether the Aura levels differ between norm/hell, but it would appear that the stats of A5 mercs, regardless of being hired in normal vs hell difficutly, are identical. I'm guessing the aura levels are identical as well.
Are all the guides below claiming "normal mercs better" just out of date? I assume they were correct in the past??
http://www.lurkerlounge.com/spirea/Hirelings/Stats.shtml
https://www.diabloii.net/forums/threads/ultimate-merc-guide-v3-0.306061/https://diablo2.diablowiki.net/Guide:Mercenary_Guide_v1.10Thank you!