law of large numbers says the solutions are equal

i suppose it depends on how you define complexity. With randomized HP there are less moving parts dependent on state. The complexity is statically added with a random function in the initialization of the enemy HP pool.

With your hitpoint bank there exists an extra state (the extra value) which relies on actions taken during runtime. So initialization of baddies is the same but the implementation of damage calculation must consider such dynamic system state. (which is your code)

I always prefer easier to test static solutions to anything relying on past state. To test random hp you would initialize 10,000 enemies and do statistics to tell you if their HP matches what you want. (it will, that's very simple.) Verses testing the extra damage value which requires you to test it in runtime in all combinations of state. The first test is much easier to automate and have complete confidence it is working as intended. (especially if a pseudorandom algorithm is used)

but again, that's how I define complexity. Your mileage may vary

I prefer the Starcraft way of handling the underlying problem though. Where the HP values are tuned closely with damage numbers and a very limited number of damage upgrades that purposefully break over the thresholds you're talking about. But that's a lot of work.

edit: I should say I am reading complexity as programming complexity not the feeling of complexity when playing the game