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I've used both an older version with a separate hood (I think there were a couple different barrel and hood styles when it came to the separate hood versions) and the current version with the sliding hood.
Optically, I didn't see much difference between the two. The newer one is supposed to have superior multi-coating, but I didn't see it making much of difference for my use.
Some users prefer the earlier version because it has a separate hood. The newer version's sliding hood is certainly annoying, in that it's constantly sliding backwards, leaving the front element vulnerable.
However, while the separate hood didn't have this problem, I found it had its own quirk - it would sometimes pop off the lens! Maybe it was just the specific lens hood I had, but needless to say, it was very annoying.
I'm usually a fan of Leica's separate lens hoods, but in the case of the early generation Noctilux, that separate hood didn't know how to stay on its lens!
If you're willing to go slightly wider & are looking at an ultra-speed lens, then consider the Voigtlander 35mm Nokton (f1.2). Its stock hood, as well as optional vented hood, are sturdy, metal, and don't pop off (or slide back, obviously).
I've found the slightly wider angle to be a bit more useful (I tend to use a super speed lens indoors), the slightly smaller f stop is effectively the same (can usually hand hold a 35mm lens about one shutter speed slower than a 50mm), and you get slightly more apparent depth of field (due to the smaller f stop & marginally wider angle) which makes focusing a bit more forgiving. Oh, and it vignettes less than the Noctilux wide open too.