++++ CAUTION:: The Officio Inquisitorus suspects the contents of this blog post may be heretical; Read at risk of termination by the Ordo Hereticus. ++++
Warhammer 40k preceded Starcraft, and indeed inspired it. The devs have explicitly stated that the Starcraft Marine is based on the Space Marines (even if they're much, much less capable. WH40k marines are genetically engineered fighting machines, so tough they're actually bulletproof when unarmored, wearing 3/4 of a tank. Starcraft marines are usually convicts who've been "resocialized" (brainwashed) for service, and are functionally comparable to the Imperial Guard in their use; mass waves of expendable troops). And the Zerg, Starcraft's ravenous assimilating spacebugs? Directly based on the Tyranids, the ravenous assimilating spacebugs from WH40k.
In the Imperium, technology is barely understood. Humans pray to the machine spirits of anything technological they use, and believe any enemy tech is heretical because machine spirits only serve man. There are entire worlds where the tech level is barely medieval, despite being part of an interstellar empire and having direct contact with spacecraft when making tithes to the Imperium. The few people who DO understand technology still worship it, and won't share with anyone. Almost all tech that actually works is from before the current Dark Age of Technology, and nobody can figure out how it works anymore (despite being at a level of complexity that could probably be completely reverse engineered by any competent modern day engineer). In short, "Stupid grimdark future".
On the other hand, Terran space is a rough frontier. Instead of a fanatical spacefaring cult, it's rednecks in space. Tech is well understood, but sometimes hard to come by. The government is still propaganda-tastic, and even despite brainwashing people for military service manages to be less oppressive than the Imperium. Trying to learn how something works and make an improved version won't get you purged for heresy, and might even make you loads of money, rather than just being required of you as a measure of near-slavery if you're in the right place to be allowed to do such things. Supporting a cause other than fighting for the God Emperor will not get you tortured to death for heresy. Actually, the things that are considered heretical are really pretty small in Terran space.
So yeah, I think I actually prefer Starcraft fluff, even if the human forces don't get as many laser beams. Power armor is widely available to the common man, and you can be pretty much assured that your standard of living will be at least TL8. Teaming up with friendly aliens to save everyone everywhere will not get you tortured for heresy. There are even actual good guys, instead of just people who are slightly less of an asshole by comparison to everyone else.