I have thought about using vehicle stealth, but I figured it would be more suited for only certain situations.
The Defense slot I fill with composite armor because it seems to help protect against so many threats and unlike some of the options it helps you the very moment you're in an engagement which is where it's needed the most IMO. It works wonders when playing solo or in group. It's the perfect command vehicle tool because it tells all the guys around you where the threats are that they cannot see. The Utility (unless I'm playing Infiltrator) I always use is the scout radar.