Mr Thin said:
Depends.
Which is cooler? The Imperial Guard. Simply because they exist in the WH40K universe.
Which would I rather join? The Starship Troopers, because I'd actually have a chance to survive. When the shit hits the fan, billions of Imperial Guard can and will be sacrificed to ensure victory.
The Starship troopers are not much better in that respect.
First battle in the movie? 100,000 died to the bugs in an HOUR, with their commanders thinking that a Zerg rush was the best thing that could counter what basically WAS the Zerg.
At least the Imperial Guard have a good chance of having some combined arms (aka armor and air support) and use descent tactics at least some of the time, so your chances to survive. You also get a free book of comedy/toilet paper in the form of the The Imperial Infantryman's Uplifting Primer.
God Emperor help you if your a Planetary Defense Force, though.