I think Warriors and Marines, just as in the rules and fluff, should have completely equal defensive capabilities, with te Warriors getting the edge at close range, and the Marine getting it at long range.

Where's the option for "tactical advance up the hill, taking cover when nesseccary"?

Looks cool. You should make up some house rules for something half way in between a Tyranid Warrior and Zoanthrope for 'em.