I say Gremlins, for no other reason then Freikorps are looking to be obscenely popular as of right now. XD Could just be my neck of the woods, but with Freikorps you'll feel like just another "Marine player" where as Goblins will be far more unique.
Grab whatever catches your fancy most though, your satisfaction in your crew is far more important then simply being unique. I'm also highly considering picking up Gremlins as my second force so I'm a tad bias. =P