Personally, I love them both. And since you're going to be grinding pillars anyway, you might as well put some stardust fragments into both. They both have features that make them really interesting and useful for a variety of situations.
~STARDUST DRAGON~
+ Can
ing FLY THROUGH WALLS AND KILL ANYONE. ANYWHERE. ANYTIME.
+ Can do insane amounts of DPS, even without making him incredibly large.
+ Because of this, as well as single-target focus, Stardust Dragon can be brutal against bosses and certain tough enemies.
- Can only target one enemy at a time, since he's a singular entity.
- Because he can fly through walls, he can frequently get distracted trying to kill enemies that otherwise are a non-issue. You don't need him to kill that enemy trapped down in the cave hidden under your spawn. You need him up here fighting the Martian Saucer, dammit.
~STARDUST CELL~
- Cannot fly through walls or target enemies through walls.
+ Can target multiple enemies due to being multiple separate minions.
+ Fire additional shots at enemies the Summoner (or his Stardust Guardian) is attacking at times.
+ Because they can't attack or target enemies through walls, they won't get distracted like the Dragon, and will only target enemies that are a direct threat to the Summoner. This is a pretty big plus in my opinion.
+ Stardust Cell projectile attacks leave a mini-cell latched onto the target that deals an ongoing 10 damage per tick. Stardust Cells can stack additional mini-cells on the target, raising the damage per tick by 10 for each additional cell. Mini-cells tick 30 times before vanishing. There appears to be no limit to how many mini-cells can be stacked onto a single target, although the firing rate is a self-imposed limit.
Then we get to the Stardust Guardian. This guy is a bit weird. You double tap down and he goes to where your cursor is and does clapping explosions to enemies that go near it, which doesn't actually do much damage. It's apparently supposed to draw aggro or something.
The aggro draw is actually pretty good but not gauranteed.