What a question! That's like asking if beer or lager is best!!
They're both good, so let's think about the differences!
The finish is different - Stahlwille are genuine Satin, Hazet are Matte, with polished highlights. I have Stahlwille I bought 20 years ago, and I find the finish is very durable. Provided you don't abuse the tool it even seems to polish slightly and almost improve with use! My Hazet is not as old, but so far there are no issues! Unlike a highly polished finish, the Matte takes use very well!
The patterns of the tools are slightly different! Stahlwille put an offset on the box end of their long combo wrenches, but not the short! Hazet the other way around! There are many other differences, and both makers offer patterns that the other doesn't!
Personally, I think the differences are quite slight, and come down to choosing the pattern you like the most! I have Stahlwille Corona (shallow offset) box end wrenches and Motor (open enders) in my main box. These are my go to tools for Metric sizes and I love 'em! But... when it come to combo wrenches I prefer the long Hazet's
Most of my German ratchets are Stahlwille, but they are mostly the durable steel handle versions which Hazet don't offer! Ditto extensions (I love the smooth pattern of the Stahlwille's which don't hang up on pipes and stuff) and some sockets (once the initial paint protection has rubbed off, the internal chroming seems really durable - I have a 10mm 6 pointer that's seen mega use and still looks good)
Most of my Inhex bits are Hazet. I think they are one of the few firms to offer two different styles of the same tool - either the forged one piece or the two piece with the gold bits! Both versions seem to be superb! I'm buying their torx too!
So, all in all, I think both are good, and it just comes down to choosing the style you like the best!
One word of caution though - I'm in the U.K. and can buy either Stahlwille or Hazet for considerably less money than say Snap On, and have no hesitation in doing so! But... if I was in the U.S. and Stahlwille / Hazet was Snap On money it would be a much harder call!