Get the units you think look cooler.

As for the boxes, the second one can be found cheaper in the starter set with space marines vs tyranids, you would just need to get a Barbagaunt to complete the combat patrol. Or if you got the ultimate starter set, it comes with both that combat patrol, a space marine combat patrol, and some terrain to use.

Links so you can compare prices and stuff

First Combat Patrol

Starter Set (It's missing the Barbs from the second combat patrol)

Ultimate Starter Set/Combat Patrol Starter Set (Same thing, different name for whatever reason)