Let me get this out of the way first - I'm well-versed in both 40k and Halo (probably more of a 40k guy). To explain this, I'l give you my stats on both universes. 40k - 3 hand-crafted armies (impguard, grey knights, tau), several Codexes, large book collection, DoW 2 + DC, etc. Halo - every Halo game except for that trash top-down one(not halo wars) (and ofc halo 5), all bungie-era Halo books, most Escalation issues + graphic novel + the Helljumpers graphic novel, etc. I'm telling you this to let you know that I know both universes better than average. I've always sort of had this argument in my head, but I saw a comment on a youtube video that nailed the difference between the two perfectly. This is not about which would win in a battle - this is about the [i][b]quality of the average soldier from each side[/b][/i]. -- 40k is hard to get a grip on in terms of consistency, but I'm pretty sure most of you would agree with me on this: Space Marines are supersoldiers, where Spartans are instruments of war. To explain: [u]The average Space Marine is a frontline soldier who often gets deployed on high-risk, full scale operations consisting of massive friendly units.[/u] A typical Space Marine operation for a usual chapter like the blood ravens or the imperial fists would be something along the lines of this: One-thousand man deployment to beat back a waaagh! on an IoM world. 100 man deployment to assault an enemy base. Several hundred man deployment to aid Imperial Guardsmen in wiping out an enemy invasion. A large operation to quell one of the many rebellions and insurrections that take place. Etc. You get the picture. Largely, Space Marines are used as what they truly are - Super soldiers. Like the difference between (when the EU still existed) an ARC trooper and a regular clone trooper. The ARC trooper is, in essence, a clone trooper who has been trained much harder and has much better equipment and creative strategies. Space Marines die - a lot. And there's more to it than simply a numbers thing. The average Space Marine will experience a military career not entirely different from that of a base Imperial Guardsman. They'll go through training, go through the scout deal, and be a frontline soldier looking to gain rank and honor. They experience many crusades and campaigns. In uncommon cases, probably once every 7-8 years after they've gained enough experience, they'll do high-gains surgical strikes, mostly to assassinate big-name xenos like warbosses. Basically, there's nothing truly [i]unique[/i] to them. [u]The average Spartan (Spartan IIs and surviving S-IIIs, mostly) is a black-operations weapon who often gets deployed on critical, low-profile operations with very small teams.[/u] Your typical Spartan will participate in almost exclusively surgical, pinprick strikes designed to dismantle enemy forces from within. This means missions like 4-8 man strike teams sent to destroy key supply networks. Two-man headhunter teams to cripple hierarchies and gather intelligence. Pinprick strikes to achieve political goals. Sabotages, etc. Generally, a Spartan mission is going to be swift, surprising and very violent. For the most part, Spartans are weapons, or instruments of war. The rate of death for Spartans is far lower than the rate of death for Space Marines. [u]And this is not a numbers thing[/u] - for example (these figures are purely thought up on the spot), what I mean by rate of death is that of all Space Marines, 45% of them will die before expected. For Spartans, 25% of them will die before expected (again, these numbers are merely placeholders to explain my point). Obviously, this isn't the same thing as simply saying that more Space Marines die. The average Spartan will experience a highly secret, highly dangerous military career. Individuality and self-sufficiency/reliance is key, as Spartans are often deployed cut off from any support for extended periods of time. They practically always engage in unconventional tactics unlike SMs: A Space Marine chapter will send 100 men in a straightforward assault to destroy an enemy fortress, whereas a 4-man Spartan squad will infiltrate the fortress, plant a bomb, and slip their way back out. Overall, I think the two are about as even as you can be when comparing two different universes, [i][b]but[/b][/i], Spartans are far more self-reliant than Space Marines. This is simply undeniable - numbers does play a part in this. Also, Spartans are much better at neutralizing emotions and are generally more 'robotic': a Space Marine will see his Chapter Master get killed by an ork warboss, start crying out and cursing, and run right into the middle of the battlefield, all angry and whatnot, yanking his helmet off, etc. That's kind of over-exaggerating it and slightly misleading ([i]slightly[/i]), but the point is that emotions come into play with Space Marines. Point, blank period. I would go on further explaining this, but that's the basic explanation. [b]EDIT: I forgot to add that Spartan-IIs are (and somewhat S-IIIs), genetically, automatically superior to Space Marines from the get-go.[/b] [b]EDIT X2: A lot of people are getting mixed up on this, and that's mostly my fault I guess, so I'll clarify Edit 1 a bit more: When I say genetically superior, I mean the fact that Spartans IIs and (I think) Spartan IIIs have to have a specific genetic makeup before they can go through the conversion process. If I'm not mistaken, it's the reason why there were so few S-IIs. In other words, you (used to) have to already be 'superhuman' before you can even hope to become a Spartan.[/b] [i]Edit x3: To make my point as clear as possible, check my comment down below. Basically, I said that put in an equal environment on equal terms, Spartans would win (imo, ofcourse)[/i]