If I could be one... I'd be running in the opposite direction screaming. The life of a space marine sounds horrible.
If I had to be one, I'd probably go with an Ultramarine quite frankly. They aren't my favorite chapter, but they are well balanced, respected, have stable geneseed, and a mini-Empire instead of a single homeworld (even if it is a mere shadow of what they had pre-Heresy). Plus, if it's in Ward's universe I'd probably be able to spit fireballs and fart lightning.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.