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Sale wise at my store Nemesis sold about 7 copies and Nemesis Lockdown sold 1 copy in 2023, so my assumption is regular Nemesis.
However Asmodee is sold out of Nemesis, meaning game stores cannot order anymore until the reprint arrives. Lockdown might still be in stock at shop asmodee.com, so take that as you will,๐
I haven't played OG Nemesis, so take this with a grain of salt... but Lockdown wasn't all that difficult to learn. The rulebook is pretty good and there are decent how to videos out there. From what I understand there are some additions that make Lockdown more complex, but if you're used to games of this complexity it's completely doable.
They're both expensive games, so I likely won't buy Nemesis, and don't really agree with buying the original for simplicity's sake prior to buying Lockdown (if you were leaning toward Lockdown). Of course if you know/think you want both at some point, go for it.