Personally, I make lists based around look. I couldn't care if it's fluffy or competitive as long as it looks good on the table. When designing lists I also consider it from the armies perspective, a marine planet-fall army intended to establish landing zones is not going to look like a standard marine force and may even come off as competitive. To me it's theme that matters most as well as overall imagery. I have little experience with sixth edition and the crappy HP rules have me sadly putting a large number of my DTs away, as well as the inability of embarked troops to capture objectives.